diff --git a/tests/Dummy/StringStream.php b/tests/Dummy/StringStream.php
new file mode 100644
index 00000000..6219f0a3
--- /dev/null
+++ b/tests/Dummy/StringStream.php
@@ -0,0 +1,81 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Dummy;
+
+use Psr\Http\Message\StreamInterface;
+
+final class StringStream implements StreamInterface
+{
+    /**
+     * @var string
+     */
+    private $value;
+
+    public function __construct(string $value)
+    {
+        $this->value = $value;
+    }
+
+    public function __toString(): string
+    {
+        return $this->value;
+    }
+
+    public function close(): void
+    {
+    }
+
+    public function detach()
+    {
+    }
+
+    public function getSize(): ?int
+    {
+    }
+
+    public function tell(): int
+    {
+    }
+
+    public function eof(): bool
+    {
+    }
+
+    public function isSeekable(): bool
+    {
+    }
+
+    public function seek(int $offset, int $whence = SEEK_SET): void
+    {
+    }
+
+    public function rewind(): void
+    {
+    }
+
+    public function isWritable(): bool
+    {
+    }
+
+    public function write(string $string): int
+    {
+    }
+
+    public function isReadable(): bool
+    {
+    }
+
+    public function read(int $length): string
+    {
+    }
+
+    public function getContents(): string
+    {
+    }
+
+    public function getMetadata(?string $key = null)
+    {
+    }
+}
\ No newline at end of file
diff --git a/tests/UnitTests/Secure/Provider/OAuthProviderTest.php b/tests/UnitTests/Secure/Provider/OAuthProviderTest.php
index 29def3e2..71a90efa 100644
--- a/tests/UnitTests/Secure/Provider/OAuthProviderTest.php
+++ b/tests/UnitTests/Secure/Provider/OAuthProviderTest.php
@@ -2,6 +2,7 @@
 
 namespace PhpTwinfield\Secure\Provider;
 
+use Dummy\StringStream;
 use GuzzleHttp\ClientInterface;
 use League\OAuth2\Client\Provider\Exception\IdentityProviderException;
 use League\OAuth2\Client\Token\AccessToken;
@@ -77,7 +78,7 @@ public function testGetAccessToken()
         $response = $this->createMock(ResponseInterface::class);
         $response->expects($this->any())
             ->method("getBody")
-            ->willReturn('{"access_token":"mock_access_token", "token_type":"bearer"}');
+            ->willReturn(new StringStream('{"access_token":"mock_access_token", "token_type":"bearer"}'));
         $response->expects($this->any())
             ->method("getHeader")
             ->willReturn(['content-type' => 'json']);
@@ -102,7 +103,7 @@ public function testExceptionThrownWhenErrorObjectReceived()
         $response = $this->createMock(ResponseInterface::class);
         $response->expects($this->any())
             ->method("getBody")
-            ->willReturn('{"error":{"type":"request","message":"someErrorMessage"}}');
+            ->willReturn(new StringStream('{"error":{"type":"request","message":"someErrorMessage"}}'));
         $response->expects($this->any())
             ->method("getHeader")
             ->willReturn(['content-type' => 'json']);
@@ -125,7 +126,7 @@ public function testGetResourceOwnerDetails()
         $response = $this->createMock(ResponseInterface::class);
         $response->expects($this->any())
             ->method("getBody")
-            ->willReturn('{"sub": "someId", "twf.organisationId": "someOrganisationId"}');
+            ->willReturn(new StringStream('{"sub": "someId", "twf.organisationId": "someOrganisationId"}'));
         $response->expects($this->any())
             ->method("getHeader")
             ->willReturn(['content-type' => 'json']);