diff --git a/src/Accessable.php b/src/Accessable.php
index ec46ab1..069da12 100644
--- a/src/Accessable.php
+++ b/src/Accessable.php
@@ -8,6 +8,8 @@
 
 namespace Art4\JsonApiClient;
 
+use Art4\JsonApiClient\Helper\AccessKey;
+
 /**
  * Accessable Interface
  */
@@ -16,7 +18,7 @@ interface Accessable
     /**
      * Get a value by a key
      *
-     * @param mixed $key The key
+     * @param int|string|AccessKey<string> $key The key
      *
      * @return mixed
      */
@@ -27,7 +29,7 @@ public function get($key);
      *
      * @deprecated `\Art4\JsonApiClient\Accessable::has()` will add `bool` as a native return type declaration in v2.0. Do the same in your implementation now to avoid errors.
      *
-     * @param mixed $key The key
+     * @param int|string|AccessKey<string> $key The key
      *
      * @return bool
      */
@@ -39,7 +41,7 @@ public function has($key);
      *
      * @deprecated `\Art4\JsonApiClient\Accessable::getKeys()` will add `array` as a native return type declaration in v2.0. Do the same in your implementation now to avoid errors.
      *
-     * @return array<string> Keys of all setted values
+     * @return array<string|int> Keys of all setted values
      */
     public function getKeys();
     // public function getKeys(): array;
diff --git a/src/Helper/AccessableTrait.php b/src/Helper/AccessableTrait.php
index 81eb8bf..d9d690a 100644
--- a/src/Helper/AccessableTrait.php
+++ b/src/Helper/AccessableTrait.php
@@ -41,7 +41,7 @@ final protected function set(string $key, $value): void
     /**
      * Returns the keys of all setted values
      *
-     * @return array<string> Keys of all setted values
+     * @return array<int|string> Keys of all setted values
      */
     final public function getKeys(): array
     {
@@ -51,7 +51,7 @@ final public function getKeys(): array
     /**
      * Check if a value exists
      *
-     * @param mixed $key The key
+     * @param int|string|AccessKey<string> $key The key
      */
     final public function has($key): bool
     {
@@ -82,7 +82,7 @@ final public function has($key): bool
     /**
      * Get a value by a key
      *
-     * @param mixed $key The key
+     * @param int|string|AccessKey<string> $key The key
      *
      * @return mixed
      */
@@ -126,7 +126,7 @@ private function getValue(string $key)
     /**
      * Parse a dot.notated.key to an object
      *
-     * @param string|AccessKey<string> $key The key
+     * @param int|string|AccessKey<string> $key The key
      *
      * @return AccessKey<string> The parsed key
      */
diff --git a/src/Serializer/ArraySerializer.php b/src/Serializer/ArraySerializer.php
index 42d1491..422037f 100644
--- a/src/Serializer/ArraySerializer.php
+++ b/src/Serializer/ArraySerializer.php
@@ -35,7 +35,7 @@ public function __construct(array $params = [])
     /**
      * Convert data in an array
      *
-     * @return array<string, mixed>|null
+     * @return array<int|string, mixed>|null
      */
     public function serialize(Accessable $data): ?array
     {
diff --git a/src/V1/Attributes.php b/src/V1/Attributes.php
index 918dcf3..9306362 100644
--- a/src/V1/Attributes.php
+++ b/src/V1/Attributes.php
@@ -8,9 +8,11 @@
 
 namespace Art4\JsonApiClient\V1;
 
-use Art4\JsonApiClient\Helper\AbstractElement;
 use Art4\JsonApiClient\Exception\AccessException;
 use Art4\JsonApiClient\Exception\ValidationException;
+use Art4\JsonApiClient\Helper\AbstractElement;
+use Art4\JsonApiClient\Helper\AccessKey;
+
 
 /**
  * Attributes Object
@@ -50,7 +52,7 @@ protected function parse($object): void
     /**
      * Get a value by the key of this object
      *
-     * @param string $key The key of the value
+     * @param int|string|AccessKey<string> $key The key of the value
      *
      * @return mixed The value
      */
diff --git a/src/V1/Document.php b/src/V1/Document.php
index ff3194d..4e971cb 100644
--- a/src/V1/Document.php
+++ b/src/V1/Document.php
@@ -9,9 +9,10 @@
 namespace Art4\JsonApiClient\V1;
 
 use Art4\JsonApiClient\Accessable;
-use Art4\JsonApiClient\Helper\AbstractElement;
 use Art4\JsonApiClient\Exception\AccessException;
 use Art4\JsonApiClient\Exception\ValidationException;
+use Art4\JsonApiClient\Helper\AbstractElement;
+use Art4\JsonApiClient\Helper\AccessKey;
 
 /**
  * Document Top Level Object
@@ -73,7 +74,7 @@ protected function parse($object): void
     /**
      * Get a value by the key of this object
      *
-     * @param string $key The key of the value
+     * @param int|string|AccessKey<string> $key The key of the value
      *
      * @return mixed The value
      */
diff --git a/src/V1/DocumentLink.php b/src/V1/DocumentLink.php
index 30ab3c3..4d0dc0b 100644
--- a/src/V1/DocumentLink.php
+++ b/src/V1/DocumentLink.php
@@ -8,9 +8,10 @@
 
 namespace Art4\JsonApiClient\V1;
 
-use Art4\JsonApiClient\Helper\AbstractElement;
 use Art4\JsonApiClient\Exception\AccessException;
 use Art4\JsonApiClient\Exception\ValidationException;
+use Art4\JsonApiClient\Helper\AbstractElement;
+use Art4\JsonApiClient\Helper\AccessKey;
 
 /**
  * Document Link Object
@@ -103,7 +104,7 @@ protected function parse($object): void
     /**
      * Get a value by the key of this object
      *
-     * @param string $key The key of the value
+     * @param int|string|AccessKey<string> $key The key of the value
      *
      * @return mixed The value
      */
diff --git a/src/V1/Error.php b/src/V1/Error.php
index 69c2f7b..346cd55 100644
--- a/src/V1/Error.php
+++ b/src/V1/Error.php
@@ -8,9 +8,10 @@
 
 namespace Art4\JsonApiClient\V1;
 
-use Art4\JsonApiClient\Helper\AbstractElement;
 use Art4\JsonApiClient\Exception\AccessException;
 use Art4\JsonApiClient\Exception\ValidationException;
+use Art4\JsonApiClient\Helper\AbstractElement;
+use Art4\JsonApiClient\Helper\AccessKey;
 
 /**
  * Error Object
@@ -105,7 +106,7 @@ protected function parse($object): void
     /**
      * Get a value by the key of this object
      *
-     * @param string $key The key of the value
+     * @param int|string|AccessKey<string> $key The key of the value
      *
      * @return mixed The value
      */
diff --git a/src/V1/ErrorCollection.php b/src/V1/ErrorCollection.php
index 42def2f..b61e0f2 100644
--- a/src/V1/ErrorCollection.php
+++ b/src/V1/ErrorCollection.php
@@ -8,9 +8,10 @@
 
 namespace Art4\JsonApiClient\V1;
 
-use Art4\JsonApiClient\Helper\AbstractElement;
 use Art4\JsonApiClient\Exception\AccessException;
 use Art4\JsonApiClient\Exception\ValidationException;
+use Art4\JsonApiClient\Helper\AbstractElement;
+use Art4\JsonApiClient\Helper\AccessKey;
 
 /**
  * Error Collection Object
@@ -44,7 +45,7 @@ protected function parse($object): void
     /**
      * Get a value by the key of this document
      *
-     * @param string $key The key of the value
+     * @param int|string|AccessKey<string> $key The key of the value
      *
      * @return mixed The value
      */
diff --git a/src/V1/ErrorLink.php b/src/V1/ErrorLink.php
index 112d09b..6c08772 100644
--- a/src/V1/ErrorLink.php
+++ b/src/V1/ErrorLink.php
@@ -8,9 +8,10 @@
 
 namespace Art4\JsonApiClient\V1;
 
-use Art4\JsonApiClient\Helper\AbstractElement;
 use Art4\JsonApiClient\Exception\AccessException;
 use Art4\JsonApiClient\Exception\ValidationException;
+use Art4\JsonApiClient\Helper\AbstractElement;
+use Art4\JsonApiClient\Helper\AccessKey;
 
 /**
  * Error Link Object
@@ -63,7 +64,7 @@ protected function parse($object): void
     /**
      * Get a value by the key of this object
      *
-     * @param string $key The key of the value
+     * @param int|string|AccessKey<string> $key The key of the value
      *
      * @return mixed The value
      */
diff --git a/src/V1/ErrorSource.php b/src/V1/ErrorSource.php
index 3c70b99..e8136e2 100644
--- a/src/V1/ErrorSource.php
+++ b/src/V1/ErrorSource.php
@@ -8,9 +8,10 @@
 
 namespace Art4\JsonApiClient\V1;
 
-use Art4\JsonApiClient\Helper\AbstractElement;
 use Art4\JsonApiClient\Exception\AccessException;
 use Art4\JsonApiClient\Exception\ValidationException;
+use Art4\JsonApiClient\Helper\AbstractElement;
+use Art4\JsonApiClient\Helper\AccessKey;
 
 /**
  * Error Source Object
@@ -52,7 +53,7 @@ protected function parse($object): void
     /**
      * Get a value by the key of this document
      *
-     * @param string $key The key of the value
+     * @param int|string|AccessKey<string> $key The key of the value
      *
      * @return mixed The value
      */
diff --git a/src/V1/Jsonapi.php b/src/V1/Jsonapi.php
index db56bef..6a5acbb 100644
--- a/src/V1/Jsonapi.php
+++ b/src/V1/Jsonapi.php
@@ -8,9 +8,10 @@
 
 namespace Art4\JsonApiClient\V1;
 
-use Art4\JsonApiClient\Helper\AbstractElement;
 use Art4\JsonApiClient\Exception\AccessException;
 use Art4\JsonApiClient\Exception\ValidationException;
+use Art4\JsonApiClient\Helper\AbstractElement;
+use Art4\JsonApiClient\Helper\AccessKey;
 
 /**
  * JSON API Object
@@ -48,7 +49,7 @@ protected function parse($object): void
     /**
      * Get a value by the key of this object
      *
-     * @param string $key The key of the value
+     * @param int|string|AccessKey<string> $key The key of the value
      *
      * @return mixed The value
      */
diff --git a/src/V1/Link.php b/src/V1/Link.php
index 1c07bae..6489924 100644
--- a/src/V1/Link.php
+++ b/src/V1/Link.php
@@ -8,9 +8,10 @@
 
 namespace Art4\JsonApiClient\V1;
 
-use Art4\JsonApiClient\Helper\AbstractElement;
 use Art4\JsonApiClient\Exception\AccessException;
 use Art4\JsonApiClient\Exception\ValidationException;
+use Art4\JsonApiClient\Helper\AbstractElement;
+use Art4\JsonApiClient\Helper\AccessKey;
 
 /**
  * Link Object
@@ -44,7 +45,7 @@ protected function parse($object): void
     /**
      * Get a value by the key of this object
      *
-     * @param string $key The key of the value
+     * @param int|string|AccessKey<string> $key The key of the value
      *
      * @return mixed The value
      */
diff --git a/src/V1/Meta.php b/src/V1/Meta.php
index 5836bab..db1dcc1 100644
--- a/src/V1/Meta.php
+++ b/src/V1/Meta.php
@@ -8,9 +8,10 @@
 
 namespace Art4\JsonApiClient\V1;
 
-use Art4\JsonApiClient\Helper\AbstractElement;
 use Art4\JsonApiClient\Exception\AccessException;
 use Art4\JsonApiClient\Exception\ValidationException;
+use Art4\JsonApiClient\Helper\AbstractElement;
+use Art4\JsonApiClient\Helper\AccessKey;
 
 /**
  * Meta Object
@@ -46,7 +47,7 @@ protected function parse($object): void
     /**
      * Get a value by the key of this object
      *
-     * @param string $key The key of the value
+     * @param int|string|AccessKey<string> $key The key of the value
      *
      * @return mixed The value
      */
diff --git a/src/V1/Relationship.php b/src/V1/Relationship.php
index 5a842cf..ec6b0a4 100644
--- a/src/V1/Relationship.php
+++ b/src/V1/Relationship.php
@@ -8,9 +8,10 @@
 
 namespace Art4\JsonApiClient\V1;
 
-use Art4\JsonApiClient\Helper\AbstractElement;
 use Art4\JsonApiClient\Exception\AccessException;
 use Art4\JsonApiClient\Exception\ValidationException;
+use Art4\JsonApiClient\Helper\AbstractElement;
+use Art4\JsonApiClient\Helper\AccessKey;
 
 /**
  * Resource Identifier Object
@@ -59,7 +60,7 @@ protected function parse($object): void
     /**
      * Get a value by the key of this object
      *
-     * @param string $key The key of the value
+     * @param int|string|AccessKey<string> $key The key of the value
      *
      * @return mixed The value
      */
diff --git a/src/V1/RelationshipCollection.php b/src/V1/RelationshipCollection.php
index 49680f0..fd9c9dc 100644
--- a/src/V1/RelationshipCollection.php
+++ b/src/V1/RelationshipCollection.php
@@ -8,9 +8,10 @@
 
 namespace Art4\JsonApiClient\V1;
 
-use Art4\JsonApiClient\Helper\AbstractElement;
 use Art4\JsonApiClient\Exception\AccessException;
 use Art4\JsonApiClient\Exception\ValidationException;
+use Art4\JsonApiClient\Helper\AbstractElement;
+use Art4\JsonApiClient\Helper\AccessKey;
 
 /**
  * Relationship Collection Object
@@ -54,7 +55,7 @@ protected function parse($object): void
     /**
      * Get a value by the key of this object
      *
-     * @param string $key The key of the value
+     * @param int|string|AccessKey<string> $key The key of the value
      *
      * @return mixed The value
      */
diff --git a/src/V1/RelationshipLink.php b/src/V1/RelationshipLink.php
index 84b1c59..3cb303f 100644
--- a/src/V1/RelationshipLink.php
+++ b/src/V1/RelationshipLink.php
@@ -11,6 +11,7 @@
 use Art4\JsonApiClient\Exception\AccessException;
 use Art4\JsonApiClient\Exception\ValidationException;
 use Art4\JsonApiClient\Helper\AbstractElement;
+use Art4\JsonApiClient\Helper\AccessKey;
 
 /**
  * Relationship Link Object
@@ -105,7 +106,7 @@ protected function parse($object): void
     /**
      * Get a value by the key of this object
      *
-     * @param string $key The key of the value
+     * @param int|string|AccessKey<string> $key The key of the value
      *
      * @return mixed The value
      */
diff --git a/src/V1/ResourceCollection.php b/src/V1/ResourceCollection.php
index 4577e2b..2b515e0 100644
--- a/src/V1/ResourceCollection.php
+++ b/src/V1/ResourceCollection.php
@@ -9,9 +9,10 @@
 namespace Art4\JsonApiClient\V1;
 
 use Art4\JsonApiClient\Accessable;
-use Art4\JsonApiClient\Helper\AbstractElement;
 use Art4\JsonApiClient\Exception\AccessException;
 use Art4\JsonApiClient\Exception\ValidationException;
+use Art4\JsonApiClient\Helper\AbstractElement;
+use Art4\JsonApiClient\Helper\AccessKey;
 
 /**
  * Resource Object
@@ -47,7 +48,7 @@ protected function parse($object): void
     /**
      * Get a value by the key of this object
      *
-     * @param string $key The key of the value
+     * @param int|string|AccessKey<string> $key The key of the value
      *
      * @return mixed The value
      */
diff --git a/src/V1/ResourceIdentifier.php b/src/V1/ResourceIdentifier.php
index 26e6b2c..df39116 100644
--- a/src/V1/ResourceIdentifier.php
+++ b/src/V1/ResourceIdentifier.php
@@ -8,9 +8,10 @@
 
 namespace Art4\JsonApiClient\V1;
 
-use Art4\JsonApiClient\Helper\AbstractElement;
 use Art4\JsonApiClient\Exception\AccessException;
 use Art4\JsonApiClient\Exception\ValidationException;
+use Art4\JsonApiClient\Helper\AbstractElement;
+use Art4\JsonApiClient\Helper\AccessKey;
 
 /**
  * Resource Identifier Object
@@ -59,7 +60,7 @@ protected function parse($object): void
     /**
      * Get a value by the key of this object
      *
-     * @param string $key The key of the value
+     * @param int|string|AccessKey<string> $key The key of the value
      *
      * @return mixed The value
      */
diff --git a/src/V1/ResourceIdentifierCollection.php b/src/V1/ResourceIdentifierCollection.php
index a887070..e820d37 100644
--- a/src/V1/ResourceIdentifierCollection.php
+++ b/src/V1/ResourceIdentifierCollection.php
@@ -8,9 +8,10 @@
 
 namespace Art4\JsonApiClient\V1;
 
-use Art4\JsonApiClient\Helper\AbstractElement;
 use Art4\JsonApiClient\Exception\AccessException;
 use Art4\JsonApiClient\Exception\ValidationException;
+use Art4\JsonApiClient\Helper\AbstractElement;
+use Art4\JsonApiClient\Helper\AccessKey;
 
 /**
  * Resource Object
@@ -42,7 +43,7 @@ protected function parse($object): void
     /**
      * Get a value by the key of this object
      *
-     * @param string $key The key of the value
+     * @param int|string|AccessKey<string> $key The key of the value
      *
      * @return mixed The value
      */
diff --git a/src/V1/ResourceItem.php b/src/V1/ResourceItem.php
index caae85a..35c43b0 100644
--- a/src/V1/ResourceItem.php
+++ b/src/V1/ResourceItem.php
@@ -8,9 +8,10 @@
 
 namespace Art4\JsonApiClient\V1;
 
-use Art4\JsonApiClient\Helper\AbstractElement;
 use Art4\JsonApiClient\Exception\AccessException;
 use Art4\JsonApiClient\Exception\ValidationException;
+use Art4\JsonApiClient\Helper\AbstractElement;
+use Art4\JsonApiClient\Helper\AccessKey;
 
 /**
  * Resource Identifier Object
@@ -77,7 +78,7 @@ protected function parse($object): void
     /**
      * Get a value by the key of this object
      *
-     * @param string $key The key of the value
+     * @param int|string|AccessKey<string> $key The key of the value
      *
      * @return mixed The value
      */
diff --git a/src/V1/ResourceItemLink.php b/src/V1/ResourceItemLink.php
index 69ce850..5315593 100644
--- a/src/V1/ResourceItemLink.php
+++ b/src/V1/ResourceItemLink.php
@@ -8,9 +8,10 @@
 
 namespace Art4\JsonApiClient\V1;
 
-use Art4\JsonApiClient\Helper\AbstractElement;
 use Art4\JsonApiClient\Exception\AccessException;
 use Art4\JsonApiClient\Exception\ValidationException;
+use Art4\JsonApiClient\Helper\AbstractElement;
+use Art4\JsonApiClient\Helper\AccessKey;
 
 /**
  * ItemLink Object
@@ -40,7 +41,7 @@ protected function parse($object): void
     /**
      * Get a value by the key of this object
      *
-     * @param string $key The key of the value
+     * @param int|string|AccessKey<string> $key The key of the value
      *
      * @return mixed The value
      */
diff --git a/src/V1/ResourceNull.php b/src/V1/ResourceNull.php
index c7df660..7876367 100644
--- a/src/V1/ResourceNull.php
+++ b/src/V1/ResourceNull.php
@@ -12,6 +12,7 @@
 use Art4\JsonApiClient\Element;
 use Art4\JsonApiClient\Manager;
 use Art4\JsonApiClient\Exception\AccessException;
+use Art4\JsonApiClient\Helper\AccessKey;
 
 /**
  * Null Resource
@@ -40,7 +41,7 @@ public function __construct($data, Manager $manager, Accessable $parent)
     /**
      * Check if a value exists in this resource
      *
-     * @param string $key The key of the value
+     * @param int|string|AccessKey<string> $key The key of the value
      *
      * @return bool false
      */
@@ -62,7 +63,7 @@ public function getKeys()
     /**
      * Get a value by the key of this identifier
      *
-     * @param string $key The key of the value
+     * @param int|string|AccessKey<string> $key The key of the value
      */
     public function get($key): void
     {
diff --git a/tests/Fixtures/HelperTrait.php b/tests/Fixtures/HelperTrait.php
index 2949bad..aa0b604 100644
--- a/tests/Fixtures/HelperTrait.php
+++ b/tests/Fixtures/HelperTrait.php
@@ -164,8 +164,8 @@ public function setUpManagerMock(): void
      *
      * @param mixed $filename
      */
-    protected function getJsonString($filename)
+    protected function getJsonString($filename): string
     {
-        return file_get_contents(__DIR__ . '/../files/' . $filename);
+        return strval(file_get_contents(__DIR__ . '/../files/' . $filename));
     }
 }
diff --git a/tests/Fixtures/V1Factory.php b/tests/Fixtures/V1Factory.php
index 282ea05..835c047 100644
--- a/tests/Fixtures/V1Factory.php
+++ b/tests/Fixtures/V1Factory.php
@@ -8,31 +8,26 @@
 
 namespace Art4\JsonApiClient\Tests\Fixtures;
 
-use Art4\JsonApiClient\Element;
 use Art4\JsonApiClient\Factory as FactoryInterface;
+use PHPUnit\Framework\TestCase;
 
 final class V1Factory implements FactoryInterface
 {
-    public $testcase;
+    public TestCase $testcase;
 
     /**
      * Create a factory
-     *
-     * @param object $testcase
-     * @param array  $args
-     *
-     * @return object
      */
-    public function __construct($testcase)
+    public function __construct(TestCase $testcase)
     {
-        return $this->testcase = $testcase;
+        $this->testcase = $testcase;
     }
 
     /**
      * Create a new instance of a class
      *
      * @param string $name
-     * @param array  $args
+     * @param array<mixed> $args
      *
      * @return object
      */
diff --git a/tests/Functional/ParsingTest.php b/tests/Functional/ParsingTest.php
index 40565e1..18a67f1 100644
--- a/tests/Functional/ParsingTest.php
+++ b/tests/Functional/ParsingTest.php
@@ -22,6 +22,8 @@ class ParsingTest extends TestCase
 
     /**
      * Provide Parser
+     *
+     * @return array<array<\Closure>>
      */
     public static function createParserProvider(): array
     {
diff --git a/tests/Functional/SerializerTest.php b/tests/Functional/SerializerTest.php
index f96c043..c835969 100644
--- a/tests/Functional/SerializerTest.php
+++ b/tests/Functional/SerializerTest.php
@@ -14,6 +14,7 @@
 use Art4\JsonApiClient\Serializer\ArraySerializer;
 use Art4\JsonApiClient\Tests\Fixtures\HelperTrait;
 use Art4\JsonApiClient\V1\Factory;
+use Exception;
 use PHPUnit\Framework\TestCase;
 
 class SerializerTest extends TestCase
@@ -22,6 +23,8 @@ class SerializerTest extends TestCase
 
     /**
      * Provide JSON API data
+     *
+     * @return array<array<string|bool>>
      */
     public static function jsonapiDataProvider(): array
     {
@@ -33,7 +36,13 @@ public static function jsonapiDataProvider(): array
             '15_create_resource_without_id.json',
         ];
 
-        foreach (glob($path . '*.json') as $file) {
+        $filenames = glob($path . '*.json');
+
+        if ($filenames === false) {
+            throw new Exception('Error while getting all json files.');
+        }
+
+        foreach ($filenames as $file) {
             $filename = str_replace($path, '', $file);
 
             // Ignore files with errors
@@ -45,9 +54,7 @@ public static function jsonapiDataProvider(): array
 
             $files[] = [
                 $filename,
-                [
-                    'is_request' => in_array($filename, $requestFiles),
-                ],
+                in_array($filename, $requestFiles),
             ];
         }
 
@@ -55,18 +62,15 @@ public static function jsonapiDataProvider(): array
     }
 
     /**
-     * @test
      * @dataProvider jsonapiDataProvider
-     *
-     * @param mixed $filename
      */
-    public function parseJsonapiDataWithErrorAbortManager($filename, array $meta): void
+    public function testParseJsonapiDataWithErrorAbortManager(string $filename, bool $isRequest): void
     {
         $manager = new ErrorAbortManager(new Factory());
 
         $string = $this->getJsonString($filename);
 
-        if ($meta['is_request']) {
+        if ($isRequest) {
             $input = new RequestStringInput($string);
         } else {
             $input = new ResponseStringInput($string);