diff --git a/.gitignore b/.gitignore
index 3be9b54..7b4c47c 100644
--- a/.gitignore
+++ b/.gitignore
@@ -3,4 +3,5 @@ build
 vendor
 composer.lock
 phpunit.xml
+.phpunit.result.cache
 .DS_Store
diff --git a/.travis.yml b/.travis.yml
index f52bc7f..4892e0f 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -12,9 +12,6 @@ env:
   global:
   - COVERAGE=0
   matrix:
-  - LARAVEL='5.5.*' TESTBENCH='3.5.*'
-  - LARAVEL='5.6.*' TESTBENCH='3.6.*'
-  - LARAVEL='5.7.*' TESTBENCH='3.7.*'
   - LARAVEL='5.8.*' TESTBENCH='3.8.*'
   - LARAVEL='6.*' TESTBENCH='4.*'
 
diff --git a/composer.json b/composer.json
index 4816204..cd02fee 100644
--- a/composer.json
+++ b/composer.json
@@ -41,14 +41,15 @@
     "socialite"
   ],
   "require": {
-    "php": ">=7.0.0",
-    "illuminate/support": "^5.5|^6.0",
-    "mollie/mollie-api-php": "^2.9"
+    "php": "^7.1.3|^7.2",
+    "illuminate/support": "^5.8.0|^6.0",
+    "mollie/mollie-api-php": "^2.9",
+    "ext-json": "*"
   },
   "require-dev": {
-    "graham-campbell/testbench": "^4.0|^5.0",
+    "graham-campbell/testbench": "^5.0|^6.0",
     "mockery/mockery": "^1.0",
-    "phpunit/phpunit": "^6.0|^7.0|^8.3",
+    "phpunit/phpunit": "^7.5|^8.0",
     "laravel/socialite": "^3.0|^4.0"
   },
   "suggest": {
diff --git a/src/MollieConnectProvider.php b/src/MollieConnectProvider.php
index dd44d30..0c14794 100644
--- a/src/MollieConnectProvider.php
+++ b/src/MollieConnectProvider.php
@@ -31,6 +31,7 @@
  */
 namespace Mollie\Laravel;
 
+use Illuminate\Support\Arr;
 use Laravel\Socialite\Two\AbstractProvider;
 use Laravel\Socialite\Two\ProviderInterface;
 use Laravel\Socialite\Two\User;
@@ -148,7 +149,7 @@ protected function getRefreshTokenFields($refresh_token)
      */
     public function getTokenFields($code)
     {
-        return array_add(parent::getTokenFields($code), 'grant_type', 'authorization_code');
+        return Arr::add(parent::getTokenFields($code), 'grant_type', 'authorization_code');
     }
 
     /**
diff --git a/tests/MollieConnectProviderTest.php b/tests/MollieConnectProviderTest.php
index e76df5d..aefb131 100644
--- a/tests/MollieConnectProviderTest.php
+++ b/tests/MollieConnectProviderTest.php
@@ -54,4 +54,20 @@ public function testExceptionIsThrownIfStateIsNotSet()
         $provider = new MollieConnectProvider($request, 'client_id', 'client_secret', 'redirect');
         $user = $provider->user();
     }
+
+    public function testGetTokenFields()
+    {
+        $request = Request::create('foo');
+        $provider = new MollieConnectProvider($request, 'client_id', 'client_secret', 'redirect');
+        $this->assertEquals(
+            [
+                'client_id' => 'client_id',
+                'client_secret' => 'client_secret',
+                'code' => 'dummy_code',
+                'redirect_uri' => 'redirect',
+                'grant_type' => 'authorization_code',
+            ],
+            $provider->getTokenFields('dummy_code')
+        );
+    }
 }