diff --git a/.gitignore b/.gitignore
index ff0b781..8bacd1c 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,6 +1,6 @@
 .idea
 .php-cs-fixer.cache
-.phpunit.result.cache
+.phpunit.cache
 composer.phar
 composer.lock
 php-cs-fixer.phar
diff --git a/composer.json b/composer.json
index ae83a95..7ed44f3 100644
--- a/composer.json
+++ b/composer.json
@@ -18,7 +18,7 @@
     "require-dev": {
         "friendsofphp/php-cs-fixer": "^3.2",
         "phpstan/phpstan": "^1.10",
-        "phpunit/phpunit": "^9"
+        "phpunit/phpunit": "^9 || ^10"
     },
     "autoload": {
         "psr-4": {
@@ -31,6 +31,7 @@
         }
     },
     "scripts": {
+        "coverage": "phpunit --coverage-html=\".phpunit.cache/code-coverage\"",
         "phpstan": "phpstan analyze --memory-limit 512M --configuration .phpstan.neon",
         "phpunit": "phpunit"
     },
diff --git a/phpunit.xml b/phpunit.xml
index 5874f45..03c0f13 100644
--- a/phpunit.xml
+++ b/phpunit.xml
@@ -1,10 +1,18 @@
 <?xml version="1.0" encoding="UTF-8"?>
-<phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" backupGlobals="false" backupStaticAttributes="false" colors="true" convertErrorsToExceptions="true" convertNoticesToExceptions="true" convertWarningsToExceptions="true" processIsolation="false" stopOnFailure="false" stopOnError="false" stopOnIncomplete="false" stopOnSkipped="false" bootstrap="vendor/autoload.php" xsi:noNamespaceSchemaLocation="https://schema.phpunit.de/9.3/phpunit.xsd">
-  <coverage>
-    <include>
-      <directory suffix=".php">src/</directory>
-    </include>
-  </coverage>
+<phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:noNamespaceSchemaLocation="https://schema.phpunit.de/10.4/phpunit.xsd"
+         bootstrap="vendor/autoload.php"
+         cacheDirectory=".phpunit.cache"
+         colors="true"
+         beStrictAboutOutputDuringTests="true"
+         displayDetailsOnIncompleteTests="true"
+         displayDetailsOnSkippedTests="true"
+         displayDetailsOnTestsThatTriggerDeprecations="true"
+         displayDetailsOnTestsThatTriggerErrors="true"
+         displayDetailsOnTestsThatTriggerNotices="true"
+         displayDetailsOnTestsThatTriggerWarnings="true"
+>
+  <coverage/>
   <testsuites>
     <testsuite name="all">
       <directory suffix="Test.php">tests/Unit/</directory>
@@ -18,4 +26,9 @@
       <directory suffix="Test.php">tests/Functional/</directory>
     </testsuite>
   </testsuites>
+  <source>
+    <include>
+      <directory suffix=".php">src/</directory>
+    </include>
+  </source>
 </phpunit>
diff --git a/tests/BC/AccessableTest.php b/tests/BC/AccessableTest.php
index 5099efc..c86f0fc 100644
--- a/tests/BC/AccessableTest.php
+++ b/tests/BC/AccessableTest.php
@@ -20,7 +20,7 @@
 namespace Art4\JsonApiClient\Tests\BC;
 
 use Art4\JsonApiClient\Accessable;
-use Art4\JsonApiClient\Tests\Fixtures\TestCase;
+use PHPUnit\Framework\TestCase;
 
 class AccessableTest extends TestCase
 {
diff --git a/tests/BC/ElementTest.php b/tests/BC/ElementTest.php
index d7de778..b0a086e 100644
--- a/tests/BC/ElementTest.php
+++ b/tests/BC/ElementTest.php
@@ -22,7 +22,7 @@
 use Art4\JsonApiClient\Accessable;
 use Art4\JsonApiClient\Element;
 use Art4\JsonApiClient\Manager;
-use Art4\JsonApiClient\Tests\Fixtures\TestCase;
+use PHPUnit\Framework\TestCase;
 
 class ElementTest extends TestCase
 {
diff --git a/tests/BC/Exception/ExceptionTest.php b/tests/BC/Exception/ExceptionTest.php
index 8ac6d9a..19bc9d0 100644
--- a/tests/BC/Exception/ExceptionTest.php
+++ b/tests/BC/Exception/ExceptionTest.php
@@ -20,7 +20,7 @@
 namespace Art4\JsonApiClient\Tests\BC\Exception;
 
 use Art4\JsonApiClient\Exception\Exception;
-use Art4\JsonApiClient\Tests\Fixtures\TestCase;
+use PHPUnit\Framework\TestCase;
 
 class ExceptionTest extends TestCase
 {
diff --git a/tests/BC/FactoryTest.php b/tests/BC/FactoryTest.php
index c747df3..71d9e15 100644
--- a/tests/BC/FactoryTest.php
+++ b/tests/BC/FactoryTest.php
@@ -20,7 +20,7 @@
 namespace Art4\JsonApiClient\Tests\BC;
 
 use Art4\JsonApiClient\Factory;
-use Art4\JsonApiClient\Tests\Fixtures\TestCase;
+use PHPUnit\Framework\TestCase;
 
 class FactoryTest extends TestCase
 {
diff --git a/tests/BC/Input/InputTest.php b/tests/BC/Input/InputTest.php
index 5ce48e0..677a665 100644
--- a/tests/BC/Input/InputTest.php
+++ b/tests/BC/Input/InputTest.php
@@ -20,7 +20,7 @@
 namespace Art4\JsonApiClient\Tests\BC\Input;
 
 use Art4\JsonApiClient\Input\Input;
-use Art4\JsonApiClient\Tests\Fixtures\TestCase;
+use PHPUnit\Framework\TestCase;
 
 class InputTest extends TestCase
 {
diff --git a/tests/BC/Input/RequestInputTest.php b/tests/BC/Input/RequestInputTest.php
index 60269a8..4c8be41 100644
--- a/tests/BC/Input/RequestInputTest.php
+++ b/tests/BC/Input/RequestInputTest.php
@@ -20,7 +20,7 @@
 namespace Art4\JsonApiClient\Tests\BC\Input;
 
 use Art4\JsonApiClient\Input\RequestInput;
-use Art4\JsonApiClient\Tests\Fixtures\TestCase;
+use PHPUnit\Framework\TestCase;
 
 class RequestInputTest extends TestCase
 {
diff --git a/tests/BC/ManagerTest.php b/tests/BC/ManagerTest.php
index 6681c03..e297289 100644
--- a/tests/BC/ManagerTest.php
+++ b/tests/BC/ManagerTest.php
@@ -21,7 +21,7 @@
 
 use Art4\JsonApiClient\Input\Input;
 use Art4\JsonApiClient\Manager;
-use Art4\JsonApiClient\Tests\Fixtures\TestCase;
+use PHPUnit\Framework\TestCase;
 
 class ManagerTest extends TestCase
 {
diff --git a/tests/BC/Serializer/SerializerTest.php b/tests/BC/Serializer/SerializerTest.php
index b07d20c..5cf7f37 100644
--- a/tests/BC/Serializer/SerializerTest.php
+++ b/tests/BC/Serializer/SerializerTest.php
@@ -21,7 +21,7 @@
 
 use Art4\JsonApiClient\Accessable;
 use Art4\JsonApiClient\Serializer\Serializer;
-use Art4\JsonApiClient\Tests\Fixtures\TestCase;
+use PHPUnit\Framework\TestCase;
 
 class SerializerTest extends TestCase
 {
diff --git a/tests/Fixtures/HelperTrait.php b/tests/Fixtures/HelperTrait.php
index b404043..4ede04f 100644
--- a/tests/Fixtures/HelperTrait.php
+++ b/tests/Fixtures/HelperTrait.php
@@ -19,6 +19,7 @@
 
 namespace Art4\JsonApiClient\Tests\Fixtures;
 
+use Art4\JsonApiClient\Accessable;
 use Art4\JsonApiClient\Factory;
 use Art4\JsonApiClient\Manager;
 use Art4\JsonApiClient\Tests\Fixtures\Factory as FixtureFactory;
@@ -28,12 +29,18 @@
  */
 trait HelperTrait
 {
+    protected Manager $manager;
+
+    protected Factory $factory;
+
+    protected Accessable $parent;
+
     /**
      * Json Values Provider
      *
      * @see http://json.org/
      */
-    public function jsonValuesProvider()
+    public static function jsonValuesProvider(): array
     {
         return [
             [new \stdClass()],
@@ -53,9 +60,9 @@ public function jsonValuesProvider()
      *
      * @see http://json.org/
      */
-    public function jsonValuesProviderWithoutObject()
+    public static function jsonValuesProviderWithoutObject(): array
     {
-        $data = $this->jsonValuesProvider();
+        $data = static::jsonValuesProvider();
 
         unset($data[0]);
 
@@ -67,9 +74,9 @@ public function jsonValuesProviderWithoutObject()
      *
      * @see http://json.org/
      */
-    public function jsonValuesProviderWithoutArray()
+    public static function jsonValuesProviderWithoutArray(): array
     {
-        $data = $this->jsonValuesProvider();
+        $data = static::jsonValuesProvider();
 
         unset($data[1]);
 
@@ -81,9 +88,9 @@ public function jsonValuesProviderWithoutArray()
      *
      * @see http://json.org/
      */
-    public function jsonValuesProviderWithoutString()
+    public static function jsonValuesProviderWithoutString(): array
     {
-        $data = $this->jsonValuesProvider();
+        $data = static::jsonValuesProvider();
 
         unset($data[2]);
 
@@ -95,9 +102,9 @@ public function jsonValuesProviderWithoutString()
      *
      * @see http://json.org/
      */
-    public function jsonValuesProviderWithoutObjectAndString()
+    public static function jsonValuesProviderWithoutObjectAndString(): array
     {
-        $data = $this->jsonValuesProvider();
+        $data = static::jsonValuesProvider();
 
         unset($data[0]);
         unset($data[2]);
@@ -110,7 +117,7 @@ public function jsonValuesProviderWithoutObjectAndString()
      *
      * @see http://json.org/
      */
-    public function jsonValuesAsStringProvider()
+    public static function jsonValuesAsStringProvider(): array
     {
         return [
             ['{}'],
@@ -130,9 +137,9 @@ public function jsonValuesAsStringProvider()
      *
      * @see http://json.org/
      */
-    public function jsonValuesAsStringProviderWithoutObject()
+    public static function jsonValuesAsStringProviderWithoutObject(): array
     {
-        $data = $this->jsonValuesAsStringProvider();
+        $data = static::jsonValuesAsStringProvider();
 
         unset($data[0]);
 
diff --git a/tests/Fixtures/TestCase.php b/tests/Fixtures/TestCase.php
deleted file mode 100644
index e549ab1..0000000
--- a/tests/Fixtures/TestCase.php
+++ /dev/null
@@ -1,45 +0,0 @@
-<?php
-/*
- * A PHP Library to handle a JSON API body in an OOP way.
- * Copyright (C) 2015-2021  Artur Weigandt  https://wlabs.de/kontakt
-
- * This program is free software: you can redistribute it and/or modify
- * it under the terms of the GNU General Public License as published by
- * the Free Software Foundation, either version 3 of the License, or
- * (at your option) any later version.
- *
- * This program is distributed in the hope that it will be useful,
- * but WITHOUT ANY WARRANTY; without even the implied warranty of
- * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
- * GNU General Public License for more details.
- *
- * You should have received a copy of the GNU General Public License
- * along with this program.  If not, see <http://www.gnu.org/licenses/>.
- */
-
-namespace Art4\JsonApiClient\Tests\Fixtures;
-
-#[\AllowDynamicProperties]
-class TestCase extends \PHPUnit\Framework\TestCase
-{
-    /**
-     * Shim for PHPUnit 6
-     *
-     * @param mixed  $exceptionName
-     * @param string $exceptionMessage
-     * @param int    $exceptionCode
-     */
-    public function setExpectedException($exceptionName, $exceptionMessage = '', $exceptionCode = null)
-    {
-        if (is_callable('parent::setExpectedException')) {
-            return parent::setExpectedException($exceptionName, $exceptionMessage, $exceptionCode);
-        }
-
-        $this->expectException($exceptionName);
-        $this->expectExceptionMessage($exceptionMessage);
-
-        if ($exceptionCode !== null) {
-            $this->expectExceptionCode($exceptionCode);
-        }
-    }
-}
diff --git a/tests/Functional/DotNotationTest.php b/tests/Functional/DotNotationTest.php
index 91f936f..8e2287f 100644
--- a/tests/Functional/DotNotationTest.php
+++ b/tests/Functional/DotNotationTest.php
@@ -25,8 +25,9 @@
 use Art4\JsonApiClient\Manager\ErrorAbortManager;
 use Art4\JsonApiClient\Tests\Fixtures\HelperTrait;
 use Art4\JsonApiClient\V1\Factory;
+use PHPUnit\Framework\TestCase;
 
-class DotNotationTest extends \Art4\JsonApiClient\Tests\Fixtures\TestCase
+class DotNotationTest extends TestCase
 {
     use HelperTrait;
 
diff --git a/tests/Functional/ErrorParsingTest.php b/tests/Functional/ErrorParsingTest.php
index 1937037..176bb62 100644
--- a/tests/Functional/ErrorParsingTest.php
+++ b/tests/Functional/ErrorParsingTest.php
@@ -21,8 +21,9 @@
 
 use Art4\JsonApiClient\Helper\Parser;
 use Art4\JsonApiClient\Tests\Fixtures\HelperTrait;
+use PHPUnit\Framework\TestCase;
 
-class ErrorParsingTest extends \Art4\JsonApiClient\Tests\Fixtures\TestCase
+class ErrorParsingTest extends TestCase
 {
     use HelperTrait;
 
diff --git a/tests/Functional/ParsingTest.php b/tests/Functional/ParsingTest.php
index 85a2e4b..ad1107b 100644
--- a/tests/Functional/ParsingTest.php
+++ b/tests/Functional/ParsingTest.php
@@ -23,18 +23,18 @@
 use Art4\JsonApiClient\Helper\Parser;
 use Art4\JsonApiClient\Input\ResponseStringInput;
 use Art4\JsonApiClient\Manager\ErrorAbortManager;
-use Art4\JsonApiClient\Serializer\ArraySerializer;
 use Art4\JsonApiClient\Tests\Fixtures\HelperTrait;
 use Art4\JsonApiClient\V1\Factory;
+use PHPUnit\Framework\TestCase;
 
-class ParsingTest extends \Art4\JsonApiClient\Tests\Fixtures\TestCase
+class ParsingTest extends TestCase
 {
     use HelperTrait;
 
     /**
      * Provide Parser
      */
-    public function createParserProvider()
+    public static function createParserProvider(): array
     {
         $errorAbortManagerParser = function ($string) {
             $manager = new ErrorAbortManager(new Factory());
diff --git a/tests/Functional/SerializerTest.php b/tests/Functional/SerializerTest.php
index 7010d98..732b7f1 100644
--- a/tests/Functional/SerializerTest.php
+++ b/tests/Functional/SerializerTest.php
@@ -24,8 +24,8 @@
 use Art4\JsonApiClient\Manager\ErrorAbortManager;
 use Art4\JsonApiClient\Serializer\ArraySerializer;
 use Art4\JsonApiClient\Tests\Fixtures\HelperTrait;
-use Art4\JsonApiClient\Tests\Fixtures\TestCase;
 use Art4\JsonApiClient\V1\Factory;
+use PHPUnit\Framework\TestCase;
 
 class SerializerTest extends TestCase
 {
@@ -34,7 +34,7 @@ class SerializerTest extends TestCase
     /**
      * Provide JSON API data
      */
-    public function jsonapiDataProvider()
+    public static function jsonapiDataProvider(): array
     {
         $path = str_replace('/', \DIRECTORY_SEPARATOR, __DIR__ . '/../files/');
         $files = [];
diff --git a/tests/Unit/Helper/ParserTest.php b/tests/Unit/Helper/ParserTest.php
index db0ee5d..33fed81 100644
--- a/tests/Unit/Helper/ParserTest.php
+++ b/tests/Unit/Helper/ParserTest.php
@@ -20,10 +20,9 @@
 namespace Art4\JsonApiClient\Tests\Unit\Helper;
 
 use Art4\JsonApiClient\Exception\InputException;
-use Art4\JsonApiClient\Exception\ValidationException;
 use Art4\JsonApiClient\Helper\Parser;
-use Art4\JsonApiClient\Tests\Fixtures\TestCase;
 use Art4\JsonApiClient\V1\Document;
+use PHPUnit\Framework\TestCase;
 
 class ParserTest extends TestCase
 {
diff --git a/tests/Unit/Input/RequestStringInputTest.php b/tests/Unit/Input/RequestStringInputTest.php
index 201cb6d..e0906fe 100644
--- a/tests/Unit/Input/RequestStringInputTest.php
+++ b/tests/Unit/Input/RequestStringInputTest.php
@@ -22,7 +22,7 @@
 use Art4\JsonApiClient\Exception\InputException;
 use Art4\JsonApiClient\Input\RequestStringInput;
 use Art4\JsonApiClient\Tests\Fixtures\HelperTrait;
-use Art4\JsonApiClient\Tests\Fixtures\TestCase;
+use PHPUnit\Framework\TestCase;
 
 class RequestStringInputTest extends TestCase
 {
diff --git a/tests/Unit/Input/ResponseStringInputTest.php b/tests/Unit/Input/ResponseStringInputTest.php
index aec77f4..c49e92e 100644
--- a/tests/Unit/Input/ResponseStringInputTest.php
+++ b/tests/Unit/Input/ResponseStringInputTest.php
@@ -22,7 +22,7 @@
 use Art4\JsonApiClient\Exception\InputException;
 use Art4\JsonApiClient\Input\ResponseStringInput;
 use Art4\JsonApiClient\Tests\Fixtures\HelperTrait;
-use Art4\JsonApiClient\Tests\Fixtures\TestCase;
+use PHPUnit\Framework\TestCase;
 
 class ResponseStringInputTest extends TestCase
 {
diff --git a/tests/Unit/Manager/ErrorAbortManagerTest.php b/tests/Unit/Manager/ErrorAbortManagerTest.php
index f7f2bf4..864f1e9 100644
--- a/tests/Unit/Manager/ErrorAbortManagerTest.php
+++ b/tests/Unit/Manager/ErrorAbortManagerTest.php
@@ -21,7 +21,7 @@
 
 use Art4\JsonApiClient\Factory;
 use Art4\JsonApiClient\Manager\ErrorAbortManager;
-use Art4\JsonApiClient\Tests\Fixtures\TestCase;
+use PHPUnit\Framework\TestCase;
 
 class ErrorAbortManagerTest extends TestCase
 {
diff --git a/tests/Unit/Serializer/ArraySerializerTest.php b/tests/Unit/Serializer/ArraySerializerTest.php
index 993472a..4cd21c6 100644
--- a/tests/Unit/Serializer/ArraySerializerTest.php
+++ b/tests/Unit/Serializer/ArraySerializerTest.php
@@ -21,7 +21,7 @@
 
 use Art4\JsonApiClient\Accessable;
 use Art4\JsonApiClient\Serializer\ArraySerializer;
-use Art4\JsonApiClient\Tests\Fixtures\TestCase;
+use PHPUnit\Framework\TestCase;
 
 class ArraySerializerTest extends TestCase
 {
diff --git a/tests/Unit/V1/AttributesTest.php b/tests/Unit/V1/AttributesTest.php
index 26aac28..ad04f57 100644
--- a/tests/Unit/V1/AttributesTest.php
+++ b/tests/Unit/V1/AttributesTest.php
@@ -23,8 +23,8 @@
 use Art4\JsonApiClient\Exception\AccessException;
 use Art4\JsonApiClient\Exception\ValidationException;
 use Art4\JsonApiClient\Tests\Fixtures\HelperTrait;
-use Art4\JsonApiClient\Tests\Fixtures\TestCase;
 use Art4\JsonApiClient\V1\Attributes;
+use PHPUnit\Framework\TestCase;
 
 class AttributesTest extends TestCase
 {
diff --git a/tests/Unit/V1/DocumentLinkTest.php b/tests/Unit/V1/DocumentLinkTest.php
index 591e490..548d3ae 100644
--- a/tests/Unit/V1/DocumentLinkTest.php
+++ b/tests/Unit/V1/DocumentLinkTest.php
@@ -23,8 +23,8 @@
 use Art4\JsonApiClient\Exception\AccessException;
 use Art4\JsonApiClient\Exception\ValidationException;
 use Art4\JsonApiClient\Tests\Fixtures\HelperTrait;
-use Art4\JsonApiClient\Tests\Fixtures\TestCase;
 use Art4\JsonApiClient\V1\DocumentLink;
+use PHPUnit\Framework\TestCase;
 
 class DocumentLinkTest extends TestCase
 {
diff --git a/tests/Unit/V1/DocumentTest.php b/tests/Unit/V1/DocumentTest.php
index d49b41d..f98ca05 100644
--- a/tests/Unit/V1/DocumentTest.php
+++ b/tests/Unit/V1/DocumentTest.php
@@ -23,8 +23,8 @@
 use Art4\JsonApiClient\Exception\AccessException;
 use Art4\JsonApiClient\Exception\ValidationException;
 use Art4\JsonApiClient\Tests\Fixtures\HelperTrait;
-use Art4\JsonApiClient\Tests\Fixtures\TestCase;
 use Art4\JsonApiClient\V1\Document;
+use PHPUnit\Framework\TestCase;
 
 class DocumentTest extends TestCase
 {
diff --git a/tests/Unit/V1/ErrorCollectionTest.php b/tests/Unit/V1/ErrorCollectionTest.php
index ffdb030..e9550ee 100644
--- a/tests/Unit/V1/ErrorCollectionTest.php
+++ b/tests/Unit/V1/ErrorCollectionTest.php
@@ -23,8 +23,8 @@
 use Art4\JsonApiClient\Exception\AccessException;
 use Art4\JsonApiClient\Exception\ValidationException;
 use Art4\JsonApiClient\Tests\Fixtures\HelperTrait;
-use Art4\JsonApiClient\Tests\Fixtures\TestCase;
 use Art4\JsonApiClient\V1\ErrorCollection;
+use PHPUnit\Framework\TestCase;
 
 class ErrorCollectionTest extends TestCase
 {
diff --git a/tests/Unit/V1/ErrorLinkTest.php b/tests/Unit/V1/ErrorLinkTest.php
index efb47b6..6251cba 100644
--- a/tests/Unit/V1/ErrorLinkTest.php
+++ b/tests/Unit/V1/ErrorLinkTest.php
@@ -23,8 +23,8 @@
 use Art4\JsonApiClient\Exception\AccessException;
 use Art4\JsonApiClient\Exception\ValidationException;
 use Art4\JsonApiClient\Tests\Fixtures\HelperTrait;
-use Art4\JsonApiClient\Tests\Fixtures\TestCase;
 use Art4\JsonApiClient\V1\ErrorLink;
+use PHPUnit\Framework\TestCase;
 
 class ErrorLinkTest extends TestCase
 {
diff --git a/tests/Unit/V1/ErrorSourceTest.php b/tests/Unit/V1/ErrorSourceTest.php
index ac8fdef..bc6a851 100644
--- a/tests/Unit/V1/ErrorSourceTest.php
+++ b/tests/Unit/V1/ErrorSourceTest.php
@@ -23,8 +23,8 @@
 use Art4\JsonApiClient\Exception\AccessException;
 use Art4\JsonApiClient\Exception\ValidationException;
 use Art4\JsonApiClient\Tests\Fixtures\HelperTrait;
-use Art4\JsonApiClient\Tests\Fixtures\TestCase;
 use Art4\JsonApiClient\V1\ErrorSource;
+use PHPUnit\Framework\TestCase;
 
 class ErrorSourceTest extends TestCase
 {
diff --git a/tests/Unit/V1/ErrorTest.php b/tests/Unit/V1/ErrorTest.php
index 0561f03..d17a5ee 100644
--- a/tests/Unit/V1/ErrorTest.php
+++ b/tests/Unit/V1/ErrorTest.php
@@ -23,8 +23,8 @@
 use Art4\JsonApiClient\Exception\AccessException;
 use Art4\JsonApiClient\Exception\ValidationException;
 use Art4\JsonApiClient\Tests\Fixtures\HelperTrait;
-use Art4\JsonApiClient\Tests\Fixtures\TestCase;
 use Art4\JsonApiClient\V1\Error;
+use PHPUnit\Framework\TestCase;
 
 class ErrorTest extends TestCase
 {
diff --git a/tests/Unit/V1/FactoryTest.php b/tests/Unit/V1/FactoryTest.php
index f46c391..37a37a6 100644
--- a/tests/Unit/V1/FactoryTest.php
+++ b/tests/Unit/V1/FactoryTest.php
@@ -25,8 +25,9 @@
 use Art4\JsonApiClient\Manager;
 use Art4\JsonApiClient\V1\Factory;
 use Art4\JsonApiClient\V1\ResourceNull;
+use PHPUnit\Framework\TestCase;
 
-class FactoryTest extends \Art4\JsonApiClient\Tests\Fixtures\TestCase
+class FactoryTest extends TestCase
 {
     /**
      * @test
diff --git a/tests/Unit/V1/JsonapiTest.php b/tests/Unit/V1/JsonapiTest.php
index 059c2aa..43d505f 100644
--- a/tests/Unit/V1/JsonapiTest.php
+++ b/tests/Unit/V1/JsonapiTest.php
@@ -23,8 +23,8 @@
 use Art4\JsonApiClient\Exception\AccessException;
 use Art4\JsonApiClient\Exception\ValidationException;
 use Art4\JsonApiClient\Tests\Fixtures\HelperTrait;
-use Art4\JsonApiClient\Tests\Fixtures\TestCase;
 use Art4\JsonApiClient\V1\Jsonapi;
+use PHPUnit\Framework\TestCase;
 
 class JsonapiTest extends TestCase
 {
diff --git a/tests/Unit/V1/LinkTest.php b/tests/Unit/V1/LinkTest.php
index 5389a9a..8cf0b29 100644
--- a/tests/Unit/V1/LinkTest.php
+++ b/tests/Unit/V1/LinkTest.php
@@ -23,8 +23,8 @@
 use Art4\JsonApiClient\Exception\AccessException;
 use Art4\JsonApiClient\Exception\ValidationException;
 use Art4\JsonApiClient\Tests\Fixtures\HelperTrait;
-use Art4\JsonApiClient\Tests\Fixtures\TestCase;
 use Art4\JsonApiClient\V1\Link;
+use PHPUnit\Framework\TestCase;
 
 class LinkTest extends TestCase
 {
diff --git a/tests/Unit/V1/MetaTest.php b/tests/Unit/V1/MetaTest.php
index dbc4966..5126ba8 100644
--- a/tests/Unit/V1/MetaTest.php
+++ b/tests/Unit/V1/MetaTest.php
@@ -23,8 +23,8 @@
 use Art4\JsonApiClient\Exception\AccessException;
 use Art4\JsonApiClient\Exception\ValidationException;
 use Art4\JsonApiClient\Tests\Fixtures\HelperTrait;
-use Art4\JsonApiClient\Tests\Fixtures\TestCase;
 use Art4\JsonApiClient\V1\Meta;
+use PHPUnit\Framework\TestCase;
 
 class MetaTest extends TestCase
 {
diff --git a/tests/Unit/V1/RelationshipCollectionTest.php b/tests/Unit/V1/RelationshipCollectionTest.php
index 84bc1c9..a0c8557 100644
--- a/tests/Unit/V1/RelationshipCollectionTest.php
+++ b/tests/Unit/V1/RelationshipCollectionTest.php
@@ -23,8 +23,8 @@
 use Art4\JsonApiClient\Exception\AccessException;
 use Art4\JsonApiClient\Exception\ValidationException;
 use Art4\JsonApiClient\Tests\Fixtures\HelperTrait;
-use Art4\JsonApiClient\Tests\Fixtures\TestCase;
 use Art4\JsonApiClient\V1\RelationshipCollection;
+use PHPUnit\Framework\TestCase;
 
 class RelationshipCollectionTest extends TestCase
 {
diff --git a/tests/Unit/V1/RelationshipLinkTest.php b/tests/Unit/V1/RelationshipLinkTest.php
index 18758c3..bef28da 100644
--- a/tests/Unit/V1/RelationshipLinkTest.php
+++ b/tests/Unit/V1/RelationshipLinkTest.php
@@ -23,14 +23,16 @@
 use Art4\JsonApiClient\Exception\AccessException;
 use Art4\JsonApiClient\Exception\ValidationException;
 use Art4\JsonApiClient\Tests\Fixtures\HelperTrait;
-use Art4\JsonApiClient\Tests\Fixtures\TestCase;
 use Art4\JsonApiClient\V1\ResourceIdentifierCollection;
 use Art4\JsonApiClient\V1\RelationshipLink;
+use PHPUnit\Framework\TestCase;
 
 class RelationshipLinkTest extends TestCase
 {
     use HelperTrait;
 
+    private Accessable $relationship;
+
     /**
      * @setup
      */
diff --git a/tests/Unit/V1/RelationshipTest.php b/tests/Unit/V1/RelationshipTest.php
index 4f07bf0..2621917 100644
--- a/tests/Unit/V1/RelationshipTest.php
+++ b/tests/Unit/V1/RelationshipTest.php
@@ -23,8 +23,8 @@
 use Art4\JsonApiClient\Exception\AccessException;
 use Art4\JsonApiClient\Exception\ValidationException;
 use Art4\JsonApiClient\Tests\Fixtures\HelperTrait;
-use Art4\JsonApiClient\Tests\Fixtures\TestCase;
 use Art4\JsonApiClient\V1\Relationship;
+use PHPUnit\Framework\TestCase;
 
 class RelationshipTest extends TestCase
 {
diff --git a/tests/Unit/V1/ResourceCollectionTest.php b/tests/Unit/V1/ResourceCollectionTest.php
index 4ad539d..2959c34 100644
--- a/tests/Unit/V1/ResourceCollectionTest.php
+++ b/tests/Unit/V1/ResourceCollectionTest.php
@@ -23,8 +23,8 @@
 use Art4\JsonApiClient\Exception\AccessException;
 use Art4\JsonApiClient\Exception\ValidationException;
 use Art4\JsonApiClient\Tests\Fixtures\HelperTrait;
-use Art4\JsonApiClient\Tests\Fixtures\TestCase;
 use Art4\JsonApiClient\V1\ResourceCollection;
+use PHPUnit\Framework\TestCase;
 
 class ResourceCollectionTest extends TestCase
 {
diff --git a/tests/Unit/V1/ResourceIdentifierCollectionTest.php b/tests/Unit/V1/ResourceIdentifierCollectionTest.php
index 42d7bc7..a438230 100644
--- a/tests/Unit/V1/ResourceIdentifierCollectionTest.php
+++ b/tests/Unit/V1/ResourceIdentifierCollectionTest.php
@@ -23,8 +23,8 @@
 use Art4\JsonApiClient\Exception\AccessException;
 use Art4\JsonApiClient\Exception\ValidationException;
 use Art4\JsonApiClient\Tests\Fixtures\HelperTrait;
-use Art4\JsonApiClient\Tests\Fixtures\TestCase;
 use Art4\JsonApiClient\V1\ResourceIdentifierCollection;
+use PHPUnit\Framework\TestCase;
 
 class ResourceIdentifierCollectionTest extends TestCase
 {
diff --git a/tests/Unit/V1/ResourceIdentifierTest.php b/tests/Unit/V1/ResourceIdentifierTest.php
index 272fdef..8d80e7d 100644
--- a/tests/Unit/V1/ResourceIdentifierTest.php
+++ b/tests/Unit/V1/ResourceIdentifierTest.php
@@ -23,8 +23,8 @@
 use Art4\JsonApiClient\Exception\AccessException;
 use Art4\JsonApiClient\Exception\ValidationException;
 use Art4\JsonApiClient\Tests\Fixtures\HelperTrait;
-use Art4\JsonApiClient\Tests\Fixtures\TestCase;
 use Art4\JsonApiClient\V1\ResourceIdentifier;
+use PHPUnit\Framework\TestCase;
 
 class ResourceIdentifierTest extends TestCase
 {
diff --git a/tests/Unit/V1/ResourceItemLinkTest.php b/tests/Unit/V1/ResourceItemLinkTest.php
index 4fca5ba..1930446 100644
--- a/tests/Unit/V1/ResourceItemLinkTest.php
+++ b/tests/Unit/V1/ResourceItemLinkTest.php
@@ -23,8 +23,8 @@
 use Art4\JsonApiClient\Exception\AccessException;
 use Art4\JsonApiClient\Exception\ValidationException;
 use Art4\JsonApiClient\Tests\Fixtures\HelperTrait;
-use Art4\JsonApiClient\Tests\Fixtures\TestCase;
 use Art4\JsonApiClient\V1\ResourceItemLink;
+use PHPUnit\Framework\TestCase;
 
 class ResourceItemLinkTest extends TestCase
 {
diff --git a/tests/Unit/V1/ResourceItemTest.php b/tests/Unit/V1/ResourceItemTest.php
index 48d9d1a..7c01985 100644
--- a/tests/Unit/V1/ResourceItemTest.php
+++ b/tests/Unit/V1/ResourceItemTest.php
@@ -23,8 +23,8 @@
 use Art4\JsonApiClient\Exception\AccessException;
 use Art4\JsonApiClient\Exception\ValidationException;
 use Art4\JsonApiClient\Tests\Fixtures\HelperTrait;
-use Art4\JsonApiClient\Tests\Fixtures\TestCase;
 use Art4\JsonApiClient\V1\ResourceItem;
+use PHPUnit\Framework\TestCase;
 
 class ResourceItemTest extends TestCase
 {
diff --git a/tests/Unit/V1/ResourceNullTest.php b/tests/Unit/V1/ResourceNullTest.php
index 7e7c83e..b3ca292 100644
--- a/tests/Unit/V1/ResourceNullTest.php
+++ b/tests/Unit/V1/ResourceNullTest.php
@@ -20,12 +20,10 @@
 namespace Art4\JsonApiClient\Tests\Unit\V1;
 
 use Art4\JsonApiClient\Accessable;
-use Art4\JsonApiClient\Manager;
 use Art4\JsonApiClient\Exception\AccessException;
 use Art4\JsonApiClient\Tests\Fixtures\HelperTrait;
-use Art4\JsonApiClient\Tests\Fixtures\V1Factory;
-use Art4\JsonApiClient\Tests\Fixtures\TestCase;
 use Art4\JsonApiClient\V1\ResourceNull;
+use PHPUnit\Framework\TestCase;
 
 class ResourceNullTest extends TestCase
 {