diff --git a/.github/workflows/code-quality.yml b/.github/workflows/code-quality.yml
index b511a86..b679f86 100644
--- a/.github/workflows/code-quality.yml
+++ b/.github/workflows/code-quality.yml
@@ -14,7 +14,7 @@ jobs:
     continue-on-error: ${{ matrix.experimental }}
     strategy:
       matrix:
-        php-versions: [8.0, 8.1, 8.2]
+        php-versions: [8.1, 8.2, 8.3]
         experimental: [false]
     steps:
       - uses: actions/checkout@v2
diff --git a/.gitignore b/.gitignore
index ec6c85a..da80ea4 100644
--- a/.gitignore
+++ b/.gitignore
@@ -7,4 +7,6 @@ Thumbs.db
 composer.lock
 /coverage
 
-example.php
\ No newline at end of file
+example.php
+.phpunit.result.cache
+/.phpunit.cache/
diff --git a/README.md b/README.md
index 796d74b..d05ab4b 100644
--- a/README.md
+++ b/README.md
@@ -15,8 +15,7 @@ Ultimately it is up to the software that uses xeroclient to deal with [serializa
 
 ## Dependencies
 
-* PHP 8.0 or greater
-* (Deprecated) [guzzlehttp/oauth-subscriber](https://packagist.org/packages/guzzlehttp/oauth-subscriber)
+* PHP 8.1 or greater
 * [league/oauth2-client](https://packagist.org/packages/league/oauth2-client)
 * [guzzlehttp/guzzle](https://packagist.org/packages/guzzlehttp/guzzle)
 
@@ -38,7 +37,7 @@ $provider = new \Radcliffe\Xero\XeroProvider([
 $url = $provider->getAuthorizationUrl();
 ```
 
-### Create a guzzle client from an authorization code (see above)
+### Create with a guzzle client from an authorization code (see above)
 
 ```php
 $client = \Radcliffe\Xero\XeroClient::createFromToken('my consumer key', 'my consumer secret', $code, 'authorization_code', 'accounting');
@@ -46,7 +45,7 @@ $client = \Radcliffe\Xero\XeroClient::createFromToken('my consumer key', 'my con
 $tokens = $client->getRefreshedToken();
 ```
 
-### Create a guzzle client with an access token
+### Create with a guzzle client with an access token
 
 ```php
 $client = \Radcliffe\Xero\XeroClient::createFromToken(
@@ -61,7 +60,7 @@ $client = \Radcliffe\Xero\XeroClient::createFromToken(
 );
 ```
 
-### Create a guzzle client with a refresh token
+### Create with a guzzle client with a refresh token
 
 Access tokens expire after 30 minutes so you can create a new client with a stored refresh token too.
 
@@ -88,7 +87,7 @@ try {
 		'query' => ['where' => 'Name.StartsWith("John")'],
 		'headers' => ['Accept' => 'application/json'],
 	];
-	$response = $client->get('Accounts', $options);
+	$response = $client->request('GET', 'Accounts', $options);
 
 	// Or use something like Symfony Serializer component.
 	$accounts = json_decode($response->getBody()->getContents());
@@ -98,6 +97,12 @@ try {
 
 ```
 
+### Error handling
+
+If the configured client does not have a valid Xero API URL or if an auth_token is not provided, then XeroRequestException is thrown as part of the Guzzle request.
+
+Previously XeroClient would throw an exception on instantiation, but this is no longer the case. If the initialize method is used directly, XeroClient will probably fail for other reasons.
+
 ### Use with a legacy OAuth1 application
 
 Please see the 0.2 branch and versions < 0.3.0.
diff --git a/composer.json b/composer.json
index db8e519..fac5fa3 100644
--- a/composer.json
+++ b/composer.json
@@ -2,7 +2,10 @@
   "name": "mradcliffe/xeroclient",
   "description": "Provides a Guzzle client for use with the Xero Accounting and Payroll APIs.",
   "minimum-stability": "stable",
-  "license": "MIT",
+  "license": [
+    "MIT",
+    "GPL-2.0-or-later"
+  ],
   "authors": [
     {
       "name": "Matthew Radcliffe",
@@ -21,10 +24,10 @@
     }
   },
   "require": {
-    "php": "^8",
-    "league/oauth2-client": "^2.4",
-    "guzzlehttp/oauth-subscriber": "0.6.0",
-    "ext-json": "*"
+    "php": "^8.1",
+    "league/oauth2-client": "^2",
+    "ext-json": "*",
+    "guzzlehttp/guzzle": "^7"
   },
   "require-dev": {
     "ext-openssl": "*",
diff --git a/src/Exception/InvalidOptionsException.php b/src/Exception/InvalidOptionsException.php
index 7e47c23..2a8bea4 100644
--- a/src/Exception/InvalidOptionsException.php
+++ b/src/Exception/InvalidOptionsException.php
@@ -2,6 +2,13 @@
 
 namespace Radcliffe\Xero\Exception;
 
+/**
+ * An exception to throw when the client has invalid options.
+ *
+ * @deprecated in 0.5.0 and removed in 0.6.0. Use a guzzle request exception
+ *             instead.
+ * @see \Radcliffe\Xero\Exception\XeroRequestException
+ */
 class InvalidOptionsException extends \Exception
 {
 }
diff --git a/src/Exception/XeroRequestException.php b/src/Exception/XeroRequestException.php
new file mode 100644
index 0000000..a7bb0a3
--- /dev/null
+++ b/src/Exception/XeroRequestException.php
@@ -0,0 +1,9 @@
+<?php
+
+namespace Radcliffe\Xero\Exception;
+
+use GuzzleHttp\Exception\RequestException;
+
+class XeroRequestException extends RequestException
+{
+}
diff --git a/src/XeroClient.php b/src/XeroClient.php
index 237709e..f523c61 100644
--- a/src/XeroClient.php
+++ b/src/XeroClient.php
@@ -3,13 +3,17 @@
 namespace Radcliffe\Xero;
 
 use GuzzleHttp\Client;
+use GuzzleHttp\ClientInterface;
 use GuzzleHttp\Exception\RequestException;
 use GuzzleHttp\HandlerStack;
-use GuzzleHttp\Subscriber\Oauth\Oauth1;
+use GuzzleHttp\Middleware;
 use League\OAuth2\Client\Token\AccessTokenInterface;
-use Radcliffe\Xero\Exception\InvalidOptionsException;
+use Psr\Http\Message\RequestInterface;
+use Psr\Http\Message\ResponseInterface;
+use Psr\Http\Message\UriInterface;
+use Radcliffe\Xero\Exception\XeroRequestException;
 
-class XeroClient extends Client implements XeroClientInterface
+class XeroClient implements XeroClientInterface
 {
    /**
     * A list of valid tenant guids.
@@ -23,6 +27,11 @@ class XeroClient extends Client implements XeroClientInterface
      */
     protected ?AccessTokenInterface $refreshedToken = null;
 
+    /**
+     * @var \GuzzleHttp\ClientInterface|null
+     */
+    protected ?ClientInterface $client = null;
+
     /**
      * {@inheritdoc}
      */
@@ -41,82 +50,15 @@ public static function getValidUrls(): array
     /**
      * Initialization method.
      *
-     * @param array<string,mixed> $config
-     *   The guzzle options.
-     *
-     * @throws \Radcliffe\Xero\Exception\InvalidOptionsException
-     * @see \GuzzleHttp\Client::__construct().
+     * @param \GuzzleHttp\ClientInterface $client
+     *   The guzzle client that is already fully configured for use with Xero.
+     *   This method of initializing XeroClient should never be used without a
+     *   static method because the guzzle developers mark useful methods as
+     *   derpecated for no good reason.
      */
-    public function __construct(array $config = [])
+    public function __construct(ClientInterface $client)
     {
-        $options = $config['options'] ?? [];
-        $scheme = $config['scheme'] ?? 'oauth1';
-        $auth = $scheme === 'oauth1' ? 'oauth' : null;
-
-        if (!isset($config['base_uri']) ||
-            !$config['base_uri'] ||
-            !$this->isValidUrl($config['base_uri'])) {
-            throw new InvalidOptionsException('API URL is not valid.');
-        }
-
-        if ($scheme === 'oauth1') {
-            // Backwards-compatible with oauth1.
-            if (!isset($config['consumer_key']) || !$config['consumer_key']) {
-                throw new InvalidOptionsException('Missing required parameter consumer_key');
-            }
-
-            if (!isset($config['consumer_secret']) || !$config['consumer_secret']) {
-                throw new InvalidOptionsException('Missing required parameter consumer_secret');
-            }
-
-            if ($config['application'] === 'private') {
-                $config['token'] = $config['consumer_key'];
-            }
-
-            if ($config['application'] === 'private') {
-                $config['token_secret'] = $config['consumer_secret'];
-            }
-
-            if ($config['application'] === 'private' &&
-                (!isset($config['private_key']) || !$this->isValidPrivateKey($config['private_key']))
-            ) {
-                throw new InvalidOptionsException('Missing required parameter private_key');
-            }
-
-            if ($config['application'] === 'private') {
-                $middleware = $this->getPrivateApplicationMiddleware($config);
-            } else {
-                $middleware = $this->getPublicApplicationMiddleware($config);
-            }
-        } elseif ($scheme === 'oauth2') {
-            // Use OAuth2 work flow.
-            if (!isset($config['auth_token'])) {
-                throw new InvalidOptionsException('Missing required parameter auth_token');
-            }
-            $options['headers']['Authorization'] = 'Bearer ' . $config['auth_token'];
-
-            if (isset($config['tenant'])) {
-                $options['headers']['xero-tenant-id'] = $config['tenant'];
-            }
-        } else {
-            throw new InvalidOptionsException('Invalid scheme provided');
-        }
-
-        if (isset($config['handler']) && is_a($config['handler'], '\GuzzleHttp\HandlerStack')) {
-            $stack = $config['handler'];
-        } else {
-            $stack = HandlerStack::create();
-        }
-
-        if (isset($middleware)) {
-            $stack->push($middleware);
-        }
-
-        parent::__construct($options + [
-            'base_uri' => $config['base_uri'],
-            'handler' => $stack,
-            'auth' => $auth,
-        ]);
+        $this->client = $client;
     }
 
     /**
@@ -124,76 +66,25 @@ public function __construct(array $config = [])
      */
     public function isValidUrl($base_uri): bool
     {
-        return in_array($base_uri, $this->getValidUrls()) ||
-          str_starts_with($base_uri, 'https://api.xero.com/oauth');
+        return in_array($base_uri, self::getValidUrls());
     }
 
     /**
      * {@inheritdoc}
      */
-    public function isValidPrivateKey($filename): bool
-    {
-        if ($filename && realpath($filename) && !is_dir($filename) && is_readable($filename)) {
-            return true;
-        }
-        return false;
-    }
-
-    /**
-     * @param array<string,mixed> $options
-     *   The options passed into the constructor.
-     *
-     * @return \GuzzleHttp\Subscriber\Oauth\Oauth1
-     *   OAuth1 middleware.
-     *
-     * @deprecated Deprecated since 0.2.0.
-     */
-    protected function getPublicApplicationMiddleware(array $options): Oauth1
+    public function getConnections(): array
     {
-        $oauth_options = [
-            'consumer_key' => $options['consumer_key'],
-            'consumer_secret' => $options['consumer_secret'],
-        ];
-
-        if (isset($options['token'])) {
-            $oauth_options['token'] = $options['token'];
-        }
-
-        if (isset($options['token_secret'])) {
-            $oauth_options['token_secret'] = $options['token_secret'];
-        }
-
-        if (isset($options['callback'])) {
-            $oauth_options['callback'] = $options['callback'];
-        }
-
-        if (isset($options['verifier'])) {
-            $oauth_options['verifier'] = $options['verifier'];
+        try {
+            $response = $this->request('GET', 'https://api.xero.com/connections', [
+              'Content-Type' => 'application/json',
+            ]);
+            return json_decode($response->getBody()->getContents(), true);
+        } catch (RequestException $e) {
+            if ($e->getCode() >= 400) {
+                throw $e;
+            }
+            return [];
         }
-
-        return new Oauth1($oauth_options);
-    }
-
-    /**
-     * @param array<string,mixed> $options
-     *   The options passed into the constructor.
-     *
-     * @return \GuzzleHttp\Subscriber\Oauth\Oauth1
-     *   OAuth1 middleware.
-     *
-     * @deprecated Deprecated since 0.2.0
-     */
-    protected function getPrivateApplicationMiddleware(array $options): Oauth1
-    {
-        return new Oauth1([
-            'consumer_key' => $options['consumer_key'],
-            'consumer_secret' => $options['consumer_secret'],
-            'token' => $options['token'],
-            'token_secret' => $options['token_secret'],
-            'private_key_file' => $options['private_key'],
-            'private_key_passphrase' => null,
-            'signature_method' => Oauth1::SIGNATURE_METHOD_RSA,
-        ]);
     }
 
     /**
@@ -201,77 +92,34 @@ protected function getPrivateApplicationMiddleware(array $options): Oauth1
      *
      * @throws \Radcliffe\Xero\Exception\InvalidOptionsException
      */
-    public static function getRequestToken(string $consumer_key, string $consumer_secret, array $options = []): array
+    public static function createFromConfig(array $config, array $options = []): static
     {
-        $config = [
-                'base_uri' => 'https://api.xero.com/oauth/',
-                'consumer_key' => $consumer_key,
-                'consumer_secret' => $consumer_secret,
-                'application' => 'public',
-            ] + $options;
-        $client = new static($config);
-
-        $tokens = [];
-        $response = $client->post('/RequestToken');
-        $pairs = explode('&', $response->getBody()->getContents());
-        foreach ($pairs as $pair) {
-            $split = explode('=', $pair, 2);
-            $parameter = urldecode($split[0]);
-            $tokens[$parameter] = isset($split[1]) ? urldecode($split[1]) : '';
+        if (isset($options['tenant'])) {
+            $config['headers']['xero-tenant-id'] = $options['tenant'];
         }
-        return $tokens;
-    }
-
-    /**
-     * {@inheritdoc}
-     *
-     * @throws \Radcliffe\Xero\Exception\InvalidOptionsException
-     */
-    public static function getAccessToken(
-        string $consumer_key,
-        string $consumer_secret,
-        string $token,
-        string $token_secret,
-        string $verifier,
-        array $options = []
-    ): array {
-        $config = [
-                'base_uri' => 'https://api.xero.com/oauth/',
-                'consumer_key' => $consumer_key,
-                'consumer_secret' => $consumer_secret,
-                'token' => $token,
-                'token_secret' => $token_secret,
-                'verifier' => $verifier,
-                'application' => 'public',
-            ] + $options;
 
-        $client = new static($config);
-
-        $tokens = [];
-        $response = $client->post('/AccessToken');
-        $pairs = explode('&', $response->getBody()->getContents());
-        foreach ($pairs as $pair) {
-            $split = explode('=', $pair, 2);
-            $parameter = urldecode($split[0]);
-            $tokens[$parameter] = isset($split[1]) ? urldecode($split[1]) : '';
+        if (isset($config['handler']) && is_a($config['handler'], '\GuzzleHttp\HandlerStack')) {
+            $stack = $config['handler'];
+        } else {
+            $stack = HandlerStack::create();
         }
-        return $tokens;
-    }
 
-    /**
-     * {@inheritdoc}
-     */
-    public function getConnections(): array
-    {
-        try {
-            $response = $this->get('https://api.xero.com/connections', ['Content-Type' => 'application/json']);
-            return json_decode($response->getBody()->getContents(), true);
-        } catch (RequestException $e) {
-            if ($e->getCode() >= 400) {
-                throw $e;
+        $stack->push(Middleware::mapRequest(function (RequestInterface $request) use ($options) {
+            $validUrls = array_filter(self::getValidUrls(), fn ($url) => str_starts_with($request->getUri(), $url));
+            if (empty($validUrls)) {
+                throw new XeroRequestException('API URL is not valid', $request);
             }
-            return [];
-        }
+
+            if (!isset($options['auth_token'])) {
+                throw new XeroRequestException('Missing required parameter auth_token', $request);
+            }
+            return $request->withHeader('Authorization', 'Bearer ' . $options['auth_token']);
+        }));
+
+        $client = new Client($config + [
+            'handler' => $stack,
+        ]);
+        return new static($client);
     }
 
     /**
@@ -279,7 +127,6 @@ public function getConnections(): array
      *
      * @throws \League\OAuth2\Client\Provider\Exception\IdentityProviderException
      * @throws \Radcliffe\Xero\Exception\InvalidOptionsException
-     * @throws \GuzzleHttp\Exception\ClientException
      */
     public static function createFromToken(
         string $id,
@@ -315,13 +162,9 @@ public static function createFromToken(
         }
 
         // Create a new static instance.
-        $instance = new static($options + [
-            'scheme' => 'oauth2',
-            'auth_token' => $token,
-        ]);
+        $instance = self::createFromConfig($options, ['auth_token' => $token]);
 
-        $response = $instance->get('https://api.xero.com/connections');
-        $instance->tenantIds = json_decode($response->getBody()->getContents(), true);
+        $instance->tenantIds = $instance->getConnections();
 
         if (isset($refreshedToken)) {
             $instance->refreshedToken = $refreshedToken;
@@ -330,6 +173,46 @@ public static function createFromToken(
         return $instance;
     }
 
+    /**
+     * {@inheritdoc}
+     *
+     * @throws \GuzzleHttp\Exception\GuzzleException
+     */
+    public function request(string $method, UriInterface|string $uri = '', array $options = []): ResponseInterface
+    {
+        return $this->client->request(strtoupper($method), $uri, $options);
+    }
+
+    /**
+     * {@inheritdoc}
+     *
+     * @throws \GuzzleHttp\Exception\GuzzleException
+     */
+    public function get(UriInterface|string $uri = '', array $options = []): ResponseInterface
+    {
+        return $this->request('GET', $uri, $options);
+    }
+
+    /**
+     * {@inheritdoc}
+     *
+     * @throws \GuzzleHttp\Exception\GuzzleException
+     */
+    public function post(UriInterface|string $uri = '', array $options = []): ResponseInterface
+    {
+        return $this->request('POST', $uri, $options);
+    }
+
+    /**
+     * {@inheritdoc}
+     *
+     * @throws \GuzzleHttp\Exception\GuzzleException
+     */
+    public function put(UriInterface|string $uri = '', array $options = []): ResponseInterface
+    {
+        return $this->request('PUT', $uri, $options);
+    }
+
     /**
      * Access tokens refreshed when creating an instance from a refresh token.
      *
diff --git a/src/XeroClientInterface.php b/src/XeroClientInterface.php
index 7090a36..29bde59 100644
--- a/src/XeroClientInterface.php
+++ b/src/XeroClientInterface.php
@@ -2,9 +2,11 @@
 
 namespace Radcliffe\Xero;
 
+use Psr\Http\Message\ResponseInterface;
+use Psr\Http\Message\UriInterface;
+
 interface XeroClientInterface
 {
-
     /**
      * Get a list of valid API URLs.
      *
@@ -19,71 +21,13 @@ public static function getValidUrls(): array;
      *
      * @return bool
      *   TRUE if the base uri is valid.
-     */
-    public function isValidUrl(string $base_uri): bool;
-
-    /**
-     * Check the private key file.
-     *
-     * @param string $filename
-     *   The file name of the private key.
-     * @return bool
-     *   TRUE if the private key is valid.
-     *
-     * @deprecated Deprecated since 0.2.0
-     */
-    public function isValidPrivateKey(string $filename): bool;
-
-    /**
-     * Get an unauthorized request token from the API.
-     *
-     * @param string $consumer_key
-     *   Consumer key.
-     * @param string $consumer_secret
-     *   Consumer secret.
-     * @param array<string,string> $options
-     *   An array of request options including other OAuth1 required properties depending on the application type.
      *
-     * @return array<string,string>
-     *   An associative array consisting of the following keys:
-     *   - oauth_token
-     *   - oauth_secret
+     * @deprecated in 0.5.0 and removed in 0.6.0. The check of base_uri is now
+     *             internal to XeroClient.
      *
-     * @deprecated Deprecated since 0.2.0
+     * @see XeroClient::createFromConfig().
      */
-    public static function getRequestToken(string $consumer_key, string $consumer_secret, array $options = []): array;
-
-    /**
-     * Get an access token from the API.
-     *
-     * @param string $consumer_key
-     *   Consumer key.
-     * @param string $consumer_secret
-     *   Consumer secret.
-     * @param string $token
-     *   OAuth token.
-     * @param string $token_secret
-     *   Token secret from the request token.
-     * @param string $verifier
-     *   The CSRF token provided by the API.
-     * @param array<string,string> $options
-     *   An array of request options to provide to Guzzle.
-     *
-     * @return array<string,string>
-     *   An associative array consisting of the following keys:
-     *   - oauth_token
-     *   - oauth_secret
-     *
-     * @deprecated Deprecated since 0.2.0
-     */
-    public static function getAccessToken(
-        string $consumer_key,
-        string $consumer_secret,
-        string $token,
-        string $token_secret,
-        string $verifier,
-        array $options = []
-    ): array;
+    public function isValidUrl(string $base_uri): bool;
 
     /**
      * Get connections authorized by the user.
@@ -144,4 +88,80 @@ public static function createFromToken(
         array $collaborators = [],
         string $redirectUri = ''
     ): static;
+
+    /**
+     * Creates an instance of XeroClient with guzzle configured from options.
+     *
+     * @param array<string,mixed> $config
+     *   The guzzle options.
+     * @param array<string,mixed> $options
+     *   The XeroClient Options:
+     *     - auth_token: the access or refresh token.
+     *     - tenant: an optional tenant id.
+     *
+     * @return static
+     *
+     * @see \GuzzleHttp\Client::__construct().
+     */
+    public static function createFromConfig(array $config, array $options = []): static;
+
+    /**
+     * Makes a request to the Xero API.
+     *
+     * @param string $method
+     *   The request methad.
+     * @param string|\Psr\Http\Message\UriInterface $uri
+     *   The endpoint path.
+     * @param array<string,mixed> $options
+     *   Options to pass to the http client.
+     *
+     * @return \Psr\Http\Message\ResponseInterface
+     *   The response from the http client.
+     */
+    public function request(string $method, string|UriInterface $uri = '', array $options = []): ResponseInterface;
+
+    /**
+     * Makes a GET request to the Xero API endpoint.
+     *
+     * @param string|\Psr\Http\Message\UriInterface $uri
+     *    The endpoint path.
+     * @param array<string,mixed> $options
+     *    Options to pass to the http client.
+     *
+     * @return \Psr\Http\Message\ResponseInterface
+     *    The response from the http client.
+     *
+     * @deprecated in 0.5.0 and removed in 0.6.0. Use the request method.
+     */
+    public function get(string|UriInterface $uri = '', array $options = []): ResponseInterface;
+
+    /**
+     * Makes a POST request to the Xero API endpoint.
+     *
+     * @param string|\Psr\Http\Message\UriInterface $uri
+     * The endpoint path.
+     * @param array<string,mixed> $options
+     * Options to pass to the http client.
+     *
+     * @return \Psr\Http\Message\ResponseInterface
+     * The response from the http client.
+     *
+     * @deprecated in 0.5.0 and removed in 0.6.0. Use the request method.
+     */
+    public function post(string|UriInterface $uri = '', array $options = []): ResponseInterface;
+
+    /**
+     * Makes a PUT request to the Xero API endpoint.
+     *
+     * @param string|\Psr\Http\Message\UriInterface $uri
+     * The endpoint path.
+     * @param array<string,mixed> $options
+     * Options to pass to the http client.
+     *
+     * @return \Psr\Http\Message\ResponseInterface
+     * The response from the http client.
+     *
+     * @deprecated in 0.5.0 and removed in 0.6.0. Use the request method.
+     */
+    public function put(string|UriInterface $uri = '', array $options = []): ResponseInterface;
 }
diff --git a/src/XeroHelperTrait.php b/src/XeroHelperTrait.php
index cf1a803..d6df8f5 100644
--- a/src/XeroHelperTrait.php
+++ b/src/XeroHelperTrait.php
@@ -7,21 +7,21 @@ trait XeroHelperTrait
     /**
      * Valid condition operators.
      *
-     * @var array
+     * @var string[]
      */
     protected static $conditionOperators = ['==', '!=', 'StartsWith', 'EndsWith', 'Contains', 'guid'];
 
     /**
      * The Xero API conditions for GET requests.
      *
-     * @var array
+     * @var string[]
      */
     protected $conditions = [];
 
     /**
      * Get the conditions for the request.
      *
-     * @return array
+     * @return string[]
      *   The conditions protected property.
      */
     public function getConditions()
@@ -34,7 +34,7 @@ public function getConditions()
      *
      * @param string $field
      *   The field to add the condition for.
-     * @param string $value
+     * @param string|bool $value
      *   The value to compare against.
      * @param string $operator
      *   The operator to use in the condition:
@@ -47,7 +47,7 @@ public function getConditions()
      *
      * @return $this
      */
-    public function addCondition($field, $value = '', $operator = '==')
+    public function addCondition(string $field, string|bool $value = '', string $operator = '==')
     {
         if (!in_array($operator, self::$conditionOperators)) {
             throw new \InvalidArgumentException('Invalid operator');
@@ -78,7 +78,7 @@ public function addCondition($field, $value = '', $operator = '==')
      *
      * @return $this
      */
-    public function addOperator($operator = 'AND')
+    public function addOperator(string $operator = 'AND')
     {
         if (!in_array($operator, ['AND', 'OR'])) {
             throw new \InvalidArgumentException('Invalid logical operator');
@@ -92,7 +92,7 @@ public function addOperator($operator = 'AND')
     /**
      * Compile the conditions array into a query parameter.
      *
-     * @return array
+     * @return array<string,string>
      *   An associative array that can be merged into the query options.
      */
     public function compileConditions()
@@ -112,10 +112,10 @@ public function compileConditions()
      * @param string $direction
      *   An optional direction.
      *
-     * @return array
+     * @return array<string,string>
      *   An associative array that can be merged into the query options.
      */
-    public function orderBy($field, $direction = 'ASC')
+    public function orderBy(string $field, string $direction = 'ASC')
     {
         $ret = ['order' => $field];
         if ($direction === 'DESC') {
@@ -130,10 +130,10 @@ public function orderBy($field, $direction = 'ASC')
      * @param string $request_parameters
      *  The HTTP Request parameters from the API to the web server.
      *
-     * @return array
+     * @return array<string,string>
      *  An associative array keyed by the parameter key.
      */
-    public function getRequestParameters($request_parameters)
+    public function getRequestParameters(string $request_parameters)
     {
         $ret = [];
         $parts = explode('&', $request_parameters);
diff --git a/src/XeroProvider.php b/src/XeroProvider.php
index c408b43..bb14730 100644
--- a/src/XeroProvider.php
+++ b/src/XeroProvider.php
@@ -154,7 +154,7 @@ public static function getValidScopes(?string $api = '', array $custom = []): ar
                 $scopes[] = "accounting.$type.read";
             }
             $scopes = array_merge($scopes, ['accounting.reports.read', 'accounting.journals.read']);
-        } elseif (str_starts_with($api, 'payroll')) {
+        } elseif (str_starts_with($api ?? '', 'payroll')) {
             // @todo Split the logic into au, uk, nz, and other sections as necessary.
             $types = ['employees', 'payruns', 'payslip', 'timesheets', 'settings'];
             foreach ($types as $type) {
diff --git a/tests/fixtures/dummy.cer b/tests/fixtures/dummy.cer
deleted file mode 100644
index ad8bbc9..0000000
--- a/tests/fixtures/dummy.cer
+++ /dev/null
@@ -1,15 +0,0 @@
------BEGIN CERTIFICATE-----
-MIICPjCCAaegAwIBAgIJAMxSkISNKDC5MA0GCSqGSIb3DQEBCwUAMDgxCzAJBgNV
-BAYTAlVTMQ0wCwYDVQQIDARPaGlvMRowGAYDVQQKDBFNYXR0aGV3IHJhZGNsaWZm
-ZTAeFw0xNjEwMjUwMDU5MTlaFw0yMTEwMjQwMDU5MTlaMDgxCzAJBgNVBAYTAlVT
-MQ0wCwYDVQQIDARPaGlvMRowGAYDVQQKDBFNYXR0aGV3IHJhZGNsaWZmZTCBnzAN
-BgkqhkiG9w0BAQEFAAOBjQAwgYkCgYEAsq7nWGCBaOEtGySpaVPFAkOvDTKcEb3s
-STh9Xye7kK0LtE8xM1UJqoFd4sxMlPPjJAKrSqZHkZ7y1WlYe/Sqr0ZfAwb0O9za
-/4m4Gk57Bcm29uWnlOh/iOdnZWZ73xpNDpeNthKzH+cWSVcm777uc1jIV61W0/nq
-1qql2hin66cCAwEAAaNQME4wHQYDVR0OBBYEFCGkWrHmCOY62z53wmpeY0p9oR2D
-MB8GA1UdIwQYMBaAFCGkWrHmCOY62z53wmpeY0p9oR2DMAwGA1UdEwQFMAMBAf8w
-DQYJKoZIhvcNAQELBQADgYEAXxbCTW/4MP/yrCj4kam0Ey2CNWMfzD14Ot0Jrel5
-vqixxMV4127JbI9Z7KAwinVlw/to2pXJnKOBHHkX+i3r7vZD4usymhdkqvvkVeqX
-JiynZ08E8iTMjGdFiF8pWeAzHyuJtkSsffu8PwVBC5Md3jL8wJY+pjyg1P4fbnIj
-tr8=
------END CERTIFICATE-----
diff --git a/tests/fixtures/dummy.pem b/tests/fixtures/dummy.pem
deleted file mode 100644
index 1b9ce2b..0000000
--- a/tests/fixtures/dummy.pem
+++ /dev/null
@@ -1,15 +0,0 @@
------BEGIN RSA PRIVATE KEY-----
-MIICWwIBAAKBgQCyrudYYIFo4S0bJKlpU8UCQ68NMpwRvexJOH1fJ7uQrQu0TzEz
-VQmqgV3izEyU8+MkAqtKpkeRnvLVaVh79KqvRl8DBvQ73Nr/ibgaTnsFybb25aeU
-6H+I52dlZnvfGk0Ol422ErMf5xZJVybvvu5zWMhXrVbT+erWqqXaGKfrpwIDAQAB
-AoGAXxW6IIqCcwhY03fn+xfL38+ayMAk00ApDDHWbZucMXoUn2gA+5sGIMjk6Drl
-ieJa/h+5fWkM7s4R1UVRA+2IIgf7evGIp+ogtFRKbs5JV7o/MQbRo8eBw84+vGiV
-WCGw7bFG/Kex4B2VACI6aAAYGJOPyboMNHGR3vCVR3NnuLECQQDZvfikwaFoXLEQ
-nFyGq1s0or/bT082vQffkrqf+qeu2Oqj6Qtl8J9WCJ6hhwXdUrDBo5ooGuUknSwo
-iET2sL35AkEA0hQRF7sxLd2p5EgMQodTWc/2BKaqFt8G5FP9mnEIY44K0PvV7WsZ
-qgKfGFm29sF2nmSsiSWog0sBSqvP1kTenwJAS8gDNowxednwwiA5T6Fpz8rozRIw
-NpPZnqU5kJ4zJ/ZUy5E7J1J0kgPT+dG9Z8b0v6AJ8fVSVMji6oRqxBTFOQJAPU7p
-5hkVMyN7j0mPemLv6kgMrjLNdLtF0aDJZCcUoak0cuTr+8vDw5/cOkkEYmL3cSZE
-TAoXcamUCOy3TPrwVQJAINW0AYXilqckLd5Fb53y84WVADHZxqnjQLr8qoB7ExJe
-RAnAt8Bh6s1k7b9rtsCy/zr9G52ALWcfU0b3GVl0+g==
------END RSA PRIVATE KEY-----
diff --git a/tests/fixtures/notreadable.pem b/tests/fixtures/notreadable.pem
deleted file mode 100644
index 1b9ce2b..0000000
--- a/tests/fixtures/notreadable.pem
+++ /dev/null
@@ -1,15 +0,0 @@
------BEGIN RSA PRIVATE KEY-----
-MIICWwIBAAKBgQCyrudYYIFo4S0bJKlpU8UCQ68NMpwRvexJOH1fJ7uQrQu0TzEz
-VQmqgV3izEyU8+MkAqtKpkeRnvLVaVh79KqvRl8DBvQ73Nr/ibgaTnsFybb25aeU
-6H+I52dlZnvfGk0Ol422ErMf5xZJVybvvu5zWMhXrVbT+erWqqXaGKfrpwIDAQAB
-AoGAXxW6IIqCcwhY03fn+xfL38+ayMAk00ApDDHWbZucMXoUn2gA+5sGIMjk6Drl
-ieJa/h+5fWkM7s4R1UVRA+2IIgf7evGIp+ogtFRKbs5JV7o/MQbRo8eBw84+vGiV
-WCGw7bFG/Kex4B2VACI6aAAYGJOPyboMNHGR3vCVR3NnuLECQQDZvfikwaFoXLEQ
-nFyGq1s0or/bT082vQffkrqf+qeu2Oqj6Qtl8J9WCJ6hhwXdUrDBo5ooGuUknSwo
-iET2sL35AkEA0hQRF7sxLd2p5EgMQodTWc/2BKaqFt8G5FP9mnEIY44K0PvV7WsZ
-qgKfGFm29sF2nmSsiSWog0sBSqvP1kTenwJAS8gDNowxednwwiA5T6Fpz8rozRIw
-NpPZnqU5kJ4zJ/ZUy5E7J1J0kgPT+dG9Z8b0v6AJ8fVSVMji6oRqxBTFOQJAPU7p
-5hkVMyN7j0mPemLv6kgMrjLNdLtF0aDJZCcUoak0cuTr+8vDw5/cOkkEYmL3cSZE
-TAoXcamUCOy3TPrwVQJAINW0AYXilqckLd5Fb53y84WVADHZxqnjQLr8qoB7ExJe
-RAnAt8Bh6s1k7b9rtsCy/zr9G52ALWcfU0b3GVl0+g==
------END RSA PRIVATE KEY-----
diff --git a/tests/src/Mocks/HelperTraitMock.php b/tests/src/Mocks/HelperTraitMock.php
new file mode 100644
index 0000000..7daf07c
--- /dev/null
+++ b/tests/src/Mocks/HelperTraitMock.php
@@ -0,0 +1,10 @@
+<?php
+
+namespace Radcliffe\Tests\Xero\Mocks;
+
+use Radcliffe\Xero\XeroHelperTrait;
+
+class HelperTraitMock
+{
+    use XeroHelperTrait;
+}
diff --git a/tests/src/XeroClientTest.php b/tests/src/XeroClientTest.php
index 6ae7769..5e73354 100644
--- a/tests/src/XeroClientTest.php
+++ b/tests/src/XeroClientTest.php
@@ -5,7 +5,6 @@
 use GuzzleHttp\Handler\MockHandler;
 use GuzzleHttp\HandlerStack;
 use GuzzleHttp\Psr7\Response;
-use Radcliffe\Xero\Exception\InvalidOptionsException;
 use Radcliffe\Xero\XeroClient;
 
 /**
@@ -16,90 +15,18 @@
 class XeroClientTest extends XeroClientTestBase
 {
 
-    /**
-     * @param array<string,mixed> $options
-     *   Invalid options to pass to the consructor.
-     *
-     * @dataProvider invalidOptionsExceptionProvider
-     */
-    public function testInvalidOptionsException(array $options): void
-    {
-        $this->expectException(InvalidOptionsException::class);
-        $client = new XeroClient($options);
-
-        $this->assertNull($client);
-    }
-
-    /**
-     * Asserts private application instantiation.
-     */
-    public function testPrivateApplication(): void
-    {
-        $options = $this->createConfiguration();
-        $client = new XeroClient($options);
-        $this->assertNotNull($client);
-    }
-
     /**
      * Asserts public application instantiation.
      */
     public function testPublicApplication(): void
     {
-        $options = $this->createConfiguration('accounting', 'public');
-        $client = new XeroClient($options);
+        $options = $this->createConfiguration();
+        $client = XeroClient::createFromConfig($options + [
+            'auth_token' => self::createRandomString(),
+        ]);
         $this->assertNotNull($client);
     }
 
-    /**
-     * Asserts get request token.
-     */
-    public function testGetRequestToken(): void
-    {
-        $options = $this->createConfiguration('accounting', 'public');
-        $expected = ['oauth_token' => $options['token'], 'oauth_secret' => $options['token_secret']];
-        $response = 'oauth_token=' . $options['token'] . '&oauth_secret=' . $options['token_secret'];
-        $mock = new MockHandler(
-            [
-                new Response(200, ['Content-Type' => 'application/x-www-form-urlencoded'], $response)
-            ]
-        );
-        $options['handler'] = new HandlerStack($mock);
-
-        $tokens = XeroClient::getRequestToken($options['consumer_key'], $options['consumer_secret'], $options);
-        $this->assertEquals($expected, $tokens);
-    }
-
-    /**
-     * Asserts getting an access token.
-     */
-    public function testGetAccessToken(): void
-    {
-        $expected = [
-            'oauth_token' => $this->createRandomString(),
-            'oauth_secret' => $this->createRandomString(),
-        ];
-
-        $options = $this->createConfiguration('accounting', 'public');
-        $response = 'oauth_token=' . $expected['oauth_token'] . '&oauth_secret=' . $expected['oauth_secret'];
-        $mock = new MockHandler(
-            [
-                new Response(200, ['Content-Type' => 'application/x-www-form-urlencoded'], $response)
-            ]
-        );
-        $options['callback'] = 'https://example.com';
-        $options['handler'] = new HandlerStack($mock);
-
-        $tokens = XeroClient::getAccessToken(
-            $options['consumer_key'],
-            $options['consumer_secret'],
-            $options['token'],
-            $options['token_secret'],
-            $options['verifier'],
-            $options
-        );
-        $this->assertEquals($expected, $tokens);
-    }
-
     /**
      * @param int $statusCode
      * @param array<string,string> $headers
@@ -107,7 +34,7 @@ public function testGetAccessToken(): void
      *
      * @dataProvider providerGetTest
      *
-     * @throws \Radcliffe\Xero\Exception\InvalidOptionsException
+     * @throws \Radcliffe\Xero\Exception\InvalidOptionsException|\GuzzleHttp\Exception\GuzzleException
      */
     public function testGet(int $statusCode, array $headers, string $body): void
     {
@@ -119,34 +46,14 @@ public function testGet(int $statusCode, array $headers, string $body): void
         );
         $options['handler'] = new HandlerStack($mock);
 
-        $client = new XeroClient($options);
+        $client = XeroClient::createFromConfig($options, [
+          'auth_token' => self::createRandomString(),
+        ]);
 
-        $response = $client->get('/BrandingThemes');
+        $response = $client->request('GET', 'BrandingThemes');
         $this->assertEquals(200, $response->getStatusCode());
     }
 
-    /**
-     * Tests trying to use an unreadable file.
-     */
-    public function testUnreadableFile(): void
-    {
-        $this->expectException(InvalidOptionsException::class);
-        $path = __DIR__ . '/../fixtures/notreadable.pem';
-        $options = [
-            'base_uri' => 'https://api.xero.com/api.xro/2.0/',
-            'consumer_key' => 'test',
-            'consumer_secret' => 'test',
-            'private_key' => $path,
-            'application' => 'private',
-        ];
-        chmod($path, 000);
-        $client = new XeroClient($options);
-        $this->assertNull($client);
-
-        // Test cleanup.
-        chmod($path, 0644);
-    }
-
     /**
      * Asserts that connections are returned and decoded.
      *
@@ -167,98 +74,34 @@ public function testGetConnections(int $statusCode, array $response, int $expect
               'Content-Type' => 'application/json',
             ], json_encode($response)),
         ]);
-        $client = new XeroClient([
+        $client = XeroClient::createFromConfig([
           'base_uri' => 'https://api.xero.com/connections',
-          'scheme' => 'oauth2',
-          'auth_token' => $this->createRandomString(),
           'handler' => new HandlerStack($mock),
-        ]);
+        ], ['auth_token' => self::createRandomString()]);
 
         $connections = $client->getConnections();
         $this->assertEquals($expectedCount, count($connections));
     }
 
-    /**
-     * @return array<int,mixed>
-     */
-    public function invalidOptionsExceptionProvider(): array
+    public function testWithInvalidUrl(): void
     {
-        return [
-            [[]],
-            [['base_uri' => '', 'application' => 'private']],
-            [
-                [
-                    'base_uri' => 'https://api.xero.com/api.xro/2.0/',
-                    'application' => 'private',
-                ],
-            ],
-            [
-                [
-                    'base_uri' => 'https://api.xero.com/api.xro/2.0/',
-                    'consumer_key' => '',
-                    'application' => 'private',
-                ],
-            ],
-            [
-                [
-                    'base_uri' => 'https://api.xero.com/api.xro/2.0/',
-                    'consumer_key' => 'test',
-                    'application' => 'private',
-                ],
-            ],
-            [
-                [
-                    'base_uri' => 'https://api.xero.com/api.xro/2.0/',
-                    'consumer_key' => 'test',
-                    'consumer_secret' => '',
-                    'application' => 'private',
-                ],
-            ],
-            [
-                [
-                    'base_uri' => 'https://api.xero.com/api.xro/2.0/',
-                    'consumer_key' => 'test',
-                    'consumer_secret' => 'test',
-                    'application' => 'private',
-                ],
-            ],
-            [
-                [
-                    'base_uri' => 'https://api.xero.com/api.xro/2.0/',
-                    'consumer_key' => 'test',
-                    'consumer_secret' => 'test',
-                    'private_key' => '',
-                    'application' => 'private',
-                ],
-            ],
-            [
-                [
-                    'base_uri' => 'https://api.xero.com/api.xro/2.0/',
-                    'consumer_key' => 'test',
-                    'consumer_secret' => 'test',
-                    'private_key' => 'nofile.pem',
-                    'application' => 'private',
-                ],
-            ],
-            [
-                [
-                    'base_uri' => 'https://api.xero.com/api.xro/2.0/',
-                    'consumer_key' => 'test',
-                    'consumer_secret' => 'test',
-                    'private_key' => 'testfile.pem',
-                    'application' => 'private',
-                ],
-            ],
-            [
-                [
-                    'base_uri' => 'https://api.xero.com/api.xro/2.0/',
-                    'consumer_key' => 'test',
-                    'consumer_secret' => 'test',
-                    'private_key' => __DIR__,
-                    'application' => 'private',
-                ],
-            ],
-        ];
+        $this->expectException('\Radcliffe\Xero\Exception\XeroRequestException');
+
+        $options = $this->createConfiguration();
+        $options['base_uri'] = 'https://example.com/';
+        $client = XeroClient::createFromConfig($options, [
+          'auth_token' => self::createRandomString(),
+        ]);
+        $client->request('GET', 'Accounts');
+    }
+
+    public function testWithoutAuthToken(): void
+    {
+        $this->expectException('\Radcliffe\Xero\Exception\XeroRequestException');
+
+        $options = $this->createConfiguration();
+        $client = XeroClient::createFromConfig($options, []);
+        $client->request('GET', 'Accounts');
     }
 
     /**
@@ -266,14 +109,14 @@ public function invalidOptionsExceptionProvider(): array
      *
      * @return array<int,mixed>
      */
-    public function providerGetTest(): array
+    public static function providerGetTest(): array
     {
         return [
             [
                 200,
                 ['Content-Type' => 'text/xml'],
                 '<?xml encoding="UTF-8" version="1.0"?><BrandingThemes><BrandingTheme><BrandingThemeID>' .
-                $this->createGuid() .
+                self::createGuid() .
                 '</BrandingThemeID><Name>Standard</Name><SortOrder>0</SortOrder><CreatedDateUTC>' .
                 '2010-06-29T18:16:36.27</CreatedDateUTC></BrandingTheme></BrandingThemes>',
             ]
@@ -286,20 +129,20 @@ public function providerGetTest(): array
      * @return array<string,mixed>
      *   An array of test cases and arguments.
      */
-    public function connectionsResponseProvider(): array
+    public static function connectionsResponseProvider(): array
     {
         return [
             'returns tenants' => [200, [
                 [
-                    'id' => $this->createGuid(),
-                    'tenantId' => $this->createGuid(),
+                    'id' => self::createGuid(),
+                    'tenantId' => self::createGuid(),
                     'tenantType' => 'ORGANISATION',
                     'createdDateUtc' => '2020-02-02T19:17:58.1117990',
                     'updatedDateUtc' => '2020-02-02T19:17:58.1117990',
                 ],
                 [
-                  'id' => $this->createGuid(),
-                  'tenantId' => $this->createGuid(),
+                  'id' => self::createGuid(),
+                  'tenantId' => self::createGuid(),
                   'tenantType' => 'ORGANISATION',
                   'createdDateUtc' => '2020-01-30T01:33:36.2717380',
                   'updatedDateUtc' => '2020-02-02T19:21:08.5739590',
diff --git a/tests/src/XeroClientTestBase.php b/tests/src/XeroClientTestBase.php
index 280a721..3139142 100644
--- a/tests/src/XeroClientTestBase.php
+++ b/tests/src/XeroClientTestBase.php
@@ -15,40 +15,25 @@ class XeroClientTestBase extends TestCase
      *
      * @param string $api
      *   The API to use: accounting or payroll.
-     * @param string $application
-     *   The application type: private, public or partner.
      *
      * @return array<string,string>
      *   An associative array of configuration options with the following keys:
      *   - base_uri: An API URL.
      *   - consumer_key: A 32-character long sring.
      *   - consumer_secret: A 32-character long string.
-     *   - private_key: File path to the private key.
-     *   - application: private or public.
      */
-    protected function createConfiguration(string $api = 'accounting', string $application = 'private'): array
+    protected function createConfiguration(string $api = 'accounting'): array
     {
         $base_uri = 'https://api.xero.com/payroll.xro/1.0/';
         if ($api === 'accounting') {
             $base_uri = 'https://api.xero.com/api.xro/2.0/';
         }
 
-        $options = [
+        return [
             'base_uri' => $base_uri,
             'consumer_key' => $this->createRandomString(),
             'consumer_secret' => $this->createRandomString(),
-            'application' => $application,
         ];
-
-        if ($application === 'private') {
-            $options['private_key'] = __DIR__ . DIRECTORY_SEPARATOR . '../fixtures/dummy.pem';
-        } else {
-            $options['token'] = $this->createRandomString();
-            $options['token_secret'] = $this->createRandomString();
-            $options['verifier'] = $this->createRandomString();
-        }
-
-        return $options;
     }
 
     /**
@@ -57,7 +42,7 @@ protected function createConfiguration(string $api = 'accounting', string $appli
      * @return string
      *   A globally-unique identifier.
      */
-    protected function createGuid(): string
+    public static function createGuid(): string
     {
         $hash = strtoupper(hash('ripemd128', md5(openssl_random_pseudo_bytes(100))));
         $guid = substr($hash, 0, 8) . '-' . substr($hash, 8, 4) . '-' . substr($hash, 12, 4);
@@ -74,7 +59,7 @@ protected function createGuid(): string
      * @return string
      *   A random string of characters.
      */
-    protected function createRandomString(int $length = 30): string
+    public static function createRandomString(int $length = 30): string
     {
         if ($length > 255) {
             throw new \InvalidArgumentException('Maximum number is 100.');
diff --git a/tests/src/XeroHelperTraitTest.php b/tests/src/XeroHelperTraitTest.php
index 58084c1..d01dd5f 100644
--- a/tests/src/XeroHelperTraitTest.php
+++ b/tests/src/XeroHelperTraitTest.php
@@ -2,6 +2,8 @@
 
 namespace Radcliffe\Tests\Xero;
 
+use Radcliffe\Tests\Xero\Mocks\HelperTraitMock;
+
 class XeroHelperTraitTest extends XeroClientTestBase
 {
 
@@ -17,8 +19,7 @@ class XeroHelperTraitTest extends XeroClientTestBase
      */
     public function testGetRequestParameters(string $parameters, array $expected): void
     {
-        /* @var $mock \Radcliffe\Xero\XeroHelperTrait */
-        $mock = $this->getMockForTrait('\Radcliffe\Xero\XeroHelperTrait');
+        $mock = new HelperTraitMock();
 
         $this->assertEquals($expected, $mock->getRequestParameters($parameters));
     }
@@ -39,8 +40,7 @@ public function testGetRequestParameters(string $parameters, array $expected): v
      */
     public function testAddCondition(string $field, bool|int|string $value, string $operator, array $expected): void
     {
-        /* @var $mock \Radcliffe\Xero\XeroHelperTrait */
-        $mock = $this->getMockForTrait('\Radcliffe\Xero\XeroHelperTrait');
+        $mock = new HelperTraitMock();
 
         $mock->addCondition($field, $value, $operator);
 
@@ -59,8 +59,7 @@ public function testAddCondition(string $field, bool|int|string $value, string $
      */
     public function testAddOperator(string $operator, array $expected): void
     {
-        /* @var $mock \Radcliffe\Xero\XeroHelperTrait */
-        $mock = $this->getMockForTrait('\Radcliffe\Xero\XeroHelperTrait');
+        $mock = new HelperTraitMock();
 
         $mock->addOperator($operator);
 
@@ -79,8 +78,7 @@ public function testAddOperator(string $operator, array $expected): void
      */
     public function testCompileConditions(array $conditions, array $expected): void
     {
-        /* @var $mock \Radcliffe\Xero\XeroHelperTrait */
-        $mock = $this->getMockForTrait('\Radcliffe\Xero\XeroHelperTrait');
+        $mock = new HelperTraitMock();
         foreach ($conditions as $condition) {
             $mock->addCondition($condition[0], $condition[1], $condition[2]);
         }
@@ -100,8 +98,7 @@ public function testCompileConditions(array $conditions, array $expected): void
      */
     public function testOrderBy(string $direction, array $expected): void
     {
-        /* @var $mock \Radcliffe\Xero\XeroHelperTrait */
-        $mock = $this->getMockForTrait('\Radcliffe\Xero\XeroHelperTrait');
+        $mock = new HelperTraitMock();
 
         $this->assertEquals($expected, $mock->orderBy('Name', $direction));
     }
@@ -111,8 +108,7 @@ public function testOrderBy(string $direction, array $expected): void
      */
     public function testInvalidAddCondition(): void
     {
-        /* @var $mock \Radcliffe\Xero\XeroHelperTrait */
-        $mock = $this->getMockForTrait('\Radcliffe\Xero\XeroHelperTrait');
+        $mock = new HelperTraitMock();
 
         $this->expectException(\InvalidArgumentException::class);
         $mock->addCondition('Name', 'Value', '<>');
@@ -123,8 +119,7 @@ public function testInvalidAddCondition(): void
      */
     public function testInvalidLogicalOperator(): void
     {
-        /* @var $mock \Radcliffe\Xero\XeroHelperTrait */
-        $mock = $this->getMockForTrait('\Radcliffe\Xero\XeroHelperTrait');
+        $mock = new HelperTraitMock();
 
         $this->expectException(\InvalidArgumentException::class);
         $mock->addOperator('NOT');
@@ -136,11 +131,11 @@ public function testInvalidLogicalOperator(): void
      * @return array<int,mixed>
      *   An array of test method parameters.
      */
-    public function requestParametersProvider(): array
+    public static function requestParametersProvider(): array
     {
         $test1_expected = [
-            'oauth_token' => $this->createRandomString(),
-            'oauth_verifier' => $this->createRandomString()
+            'oauth_token' => self::createRandomString(),
+            'oauth_verifier' => self::createRandomString()
         ];
         $test1_string = 'oauth_token=' . urlencode($test1_expected['oauth_token']) . '&' . 'oauth_verifier=' .
             urlencode($test1_expected['oauth_verifier']);
@@ -155,9 +150,9 @@ public function requestParametersProvider(): array
      *
      * @return array<int,mixed>
      */
-    public function addConditionProvider(): array
+    public static function addConditionProvider(): array
     {
-        $guid = $this->createGuid();
+        $guid = self::createGuid();
         return [
             ['Name', 'Test Value', '==', ['Name=="Test Value"']],
             ['Name', 'Test Value', '!=', ['Name!="Test Value"']],
@@ -173,7 +168,7 @@ public function addConditionProvider(): array
      *
      * @return array<int,mixed>
      */
-    public function addOperatorProvider(): array
+    public static function addOperatorProvider(): array
     {
         return [['AND', ['AND']], ['OR', ['OR']]];
     }
@@ -183,7 +178,7 @@ public function addOperatorProvider(): array
      *
      * @return array<int,mixed>
      */
-    public function compileConditionsProvider(): array
+    public static function compileConditionsProvider(): array
     {
         return [
             [[], []],
@@ -208,7 +203,7 @@ public function compileConditionsProvider(): array
      *
      * @return array<int,mixed>
      */
-    public function orderByProvider(): array
+    public static function orderByProvider(): array
     {
         return [
             ['ASC', ['order' => 'Name']],
diff --git a/tests/src/XeroProviderTest.php b/tests/src/XeroProviderTest.php
index 83ca7d7..055bb8a 100644
--- a/tests/src/XeroProviderTest.php
+++ b/tests/src/XeroProviderTest.php
@@ -112,20 +112,24 @@ public function validScopesProvider(): array
      *
      * @dataProvider provideResponseData
      *
-     * @throws \League\OAuth2\Client\Provider\Exception\IdentityProviderException
+     * @throws \League\OAuth2\Client\Provider\Exception\IdentityProviderException|\GuzzleHttp\Exception\GuzzleException
      */
     public function testGetResponseMessage(array $data, string $expected): void
     {
         $json = json_encode($data);
         $this->expectExceptionMessage($expected);
 
+        $streamProphet = $this->prophet->prophesize('\Psr\Http\Message\StreamInterface');
+        $streamProphet->getContents()->willReturn($json);
+        $streamProphet->__toString()->willReturn($json);
+
         $requestProphet = $this->prophet->prophesize('\Psr\Http\Message\RequestInterface');
         $responseProphet = $this->prophet->prophesize('\Psr\Http\Message\ResponseInterface');
         $responseProphet->getStatusCode()->willReturn(400);
-        $responseProphet->getBody()->willReturn($json);
+        $responseProphet->getBody()->willReturn($streamProphet->reveal());
         $responseProphet
             ->getHeader(Argument::containingString('content-type'))
-            ->willReturn('application/json');
+            ->willReturn(['application/json']);
         $guzzleProphet = $this->prophet->prophesize('\GuzzleHttp\ClientInterface');
         $guzzleProphet->send(Argument::any())->willReturn($responseProphet->reveal());