diff --git a/Build/php-cs-fixer/config.php b/Build/php-cs-fixer/config.php
index 57ebfa08..755b1ae4 100644
--- a/Build/php-cs-fixer/config.php
+++ b/Build/php-cs-fixer/config.php
@@ -56,6 +56,10 @@
         'no_unused_imports' => true,
         'no_useless_else' => true,
         'no_useless_nullsafe_operator' => true,
+        'nullable_type_declaration' => [
+            'syntax' => 'question_mark',
+        ],
+        'nullable_type_declaration_for_default_null_value' => true,
         'ordered_imports' => ['imports_order' => ['class', 'function', 'const'], 'sort_algorithm' => 'alpha'],
         'php_unit_construct' => ['assertions' => ['assertEquals', 'assertSame', 'assertNotEquals', 'assertNotSame']],
         'php_unit_mock_short_will_return' => true,
diff --git a/Classes/Core/Functional/Framework/AssignablePropertyTrait.php b/Classes/Core/Functional/Framework/AssignablePropertyTrait.php
index d32f36ed..3c752602 100644
--- a/Classes/Core/Functional/Framework/AssignablePropertyTrait.php
+++ b/Classes/Core/Functional/Framework/AssignablePropertyTrait.php
@@ -25,7 +25,7 @@ trait AssignablePropertyTrait
      * @param callable|null $cast
      * @return static
      */
-    private function assign(array $data, callable $cast = null)
+    private function assign(array $data, ?callable $cast = null)
     {
         if ($cast !== null) {
             $data = array_map($cast, $data);
diff --git a/Classes/Core/Functional/Framework/DataHandling/ActionService.php b/Classes/Core/Functional/Framework/DataHandling/ActionService.php
index 60f0def8..828073e5 100644
--- a/Classes/Core/Functional/Framework/DataHandling/ActionService.php
+++ b/Classes/Core/Functional/Framework/DataHandling/ActionService.php
@@ -111,7 +111,7 @@ public function createNewRecords(int $pageId, array $tableRecordData): array
      * modifyRecord('tt_content', 42, ['hidden' => '1']); // Modify a single record
      * modifyRecord('tt_content', 42, ['hidden' => '1'], ['tx_irre_table' => [4]]); // Modify a record and delete a child
      */
-    public function modifyRecord(string $tableName, int $uid, array $recordData, array $deleteTableRecordIds = null)
+    public function modifyRecord(string $tableName, int $uid, array $recordData, ?array $deleteTableRecordIds = null)
     {
         $dataMap = [
             $tableName => [
@@ -266,7 +266,7 @@ public function clearWorkspaceRecords(array $tableRecordIds)
      * Example:
      * copyRecord('tt_content', 42, 5, ['header' => 'Testing #1']);
      */
-    public function copyRecord(string $tableName, int $uid, int $pageId, array $recordData = null): array
+    public function copyRecord(string $tableName, int $uid, int $pageId, ?array $recordData = null): array
     {
         $commandMap = [
             $tableName => [
@@ -302,7 +302,7 @@ public function copyRecord(string $tableName, int $uid, int $pageId, array $reco
      * @param array $recordData Additional record data to change when moving.
      * @return array
      */
-    public function moveRecord(string $tableName, int $uid, int $targetUid, array $recordData = null): array
+    public function moveRecord(string $tableName, int $uid, int $targetUid, ?array $recordData = null): array
     {
         $commandMap = [
             $tableName => [
diff --git a/Classes/Core/Functional/Framework/DataHandling/CsvWriterStreamFilter.php b/Classes/Core/Functional/Framework/DataHandling/CsvWriterStreamFilter.php
index 5e4877ba..c3140c33 100644
--- a/Classes/Core/Functional/Framework/DataHandling/CsvWriterStreamFilter.php
+++ b/Classes/Core/Functional/Framework/DataHandling/CsvWriterStreamFilter.php
@@ -49,7 +49,7 @@ public static function register()
      * @param resource $stream
      * @param string $sequence
      */
-    public static function apply($stream, string $sequence = null): \Closure
+    public static function apply($stream, ?string $sequence = null): \Closure
     {
         static::register();
         if ($sequence === null) {
diff --git a/Classes/Core/Functional/Framework/DataHandling/DataSet.php b/Classes/Core/Functional/Framework/DataHandling/DataSet.php
index 70be2a73..c2a505b4 100644
--- a/Classes/Core/Functional/Framework/DataHandling/DataSet.php
+++ b/Classes/Core/Functional/Framework/DataHandling/DataSet.php
@@ -299,7 +299,7 @@ public function getElements(string $tableName)
      * @param int|null $padding
      * @deprecated Will be removed with core v12 compatible testing-framework.
      */
-    public function persist(string $fileName, int $padding = null)
+    public function persist(string $fileName, ?int $padding = null)
     {
         $fileHandle = fopen($fileName, 'w');
         $modifier = CsvWriterStreamFilter::apply($fileHandle);
@@ -330,7 +330,7 @@ public function persist(string $fileName, int $padding = null)
      * @return array
      * @deprecated Will be removed with core v12 compatible testing-framework.
      */
-    protected function pad(array $values, int $padding = null): array
+    protected function pad(array $values, ?int $padding = null): array
     {
         if ($padding === null) {
             return $values;
diff --git a/Classes/Core/Functional/Framework/DataHandling/Scenario/DataHandlerFactory.php b/Classes/Core/Functional/Framework/DataHandling/Scenario/DataHandlerFactory.php
index 3f86913c..04b8fb0d 100644
--- a/Classes/Core/Functional/Framework/DataHandling/Scenario/DataHandlerFactory.php
+++ b/Classes/Core/Functional/Framework/DataHandling/Scenario/DataHandlerFactory.php
@@ -110,8 +110,8 @@ public function getSuggestedIds(): array
      */
     private function processEntities(
         array $settings,
-        string $nodeId = null,
-        string $parentId = null
+        ?string $nodeId = null,
+        ?string $parentId = null
     ): void {
         foreach ($settings as $entityName => $entitySettings) {
             $entityConfiguration = $this->provideEntityConfiguration($entityName);
@@ -135,8 +135,8 @@ private function processEntities(
     private function processEntityItem(
         EntityConfiguration $entityConfiguration,
         array $itemSettings,
-        string $nodeId = null,
-        string $parentId = null
+        ?string $nodeId = null,
+        ?string $parentId = null
     ): void {
         $values = $this->processEntityValues(
             $entityConfiguration,
@@ -199,7 +199,7 @@ private function processLanguageVariantItem(
         EntityConfiguration $entityConfiguration,
         array $itemSettings,
         array $ancestorIds,
-        string $nodeId = null
+        ?string $nodeId = null
     ): void {
         $values = $this->processEntityValues(
             $entityConfiguration,
@@ -240,7 +240,7 @@ private function processVersionVariantItem(
         EntityConfiguration $entityConfiguration,
         array $itemSettings,
         string $ancestorId,
-        string $nodeId = null
+        ?string $nodeId = null
     ): void {
         if (isset($itemSettings['self'])) {
             throw new \LogicException(
@@ -284,8 +284,8 @@ private function processVersionVariantItem(
     private function processEntityValues(
         EntityConfiguration $entityConfiguration,
         array $itemSettings,
-        string $nodeId = null,
-        string $parentId = null
+        ?string $nodeId = null,
+        ?string $parentId = null
     ): array {
         if (isset($itemSettings['self']) && isset($itemSettings['version'])) {
             throw new \LogicException(
diff --git a/Classes/Core/Functional/Framework/Frontend/Collector.php b/Classes/Core/Functional/Framework/Frontend/Collector.php
index 062146a5..37bcafa4 100644
--- a/Classes/Core/Functional/Framework/Frontend/Collector.php
+++ b/Classes/Core/Functional/Framework/Frontend/Collector.php
@@ -52,7 +52,7 @@ class Collector implements SingletonInterface
      */
     public $cObj;
 
-    public function addRecordData($content, array $configuration = null): void
+    public function addRecordData($content, ?array $configuration = null): void
     {
         $recordIdentifier = $this->cObj->currentRecord;
         [$tableName] = explode(':', $recordIdentifier);
@@ -71,7 +71,7 @@ public function addRecordData($content, array $configuration = null): void
         }
     }
 
-    public function addFileData($content, array $configuration = null): void
+    public function addFileData($content, ?array $configuration = null): void
     {
         $currentFile = $this->cObj->getCurrentFile();
 
@@ -134,7 +134,7 @@ protected function addToStructure($levelIdentifier, $recordIdentifier, array $re
      * @param string $content
      * @param array|null $configuration
      */
-    public function attachSection($content, array $configuration = null): void
+    public function attachSection($content, ?array $configuration = null): void
     {
         $section = [
             'structure' => $this->structure,
diff --git a/Classes/Core/Functional/Framework/Frontend/Renderer.php b/Classes/Core/Functional/Framework/Frontend/Renderer.php
index e1eba67b..a53fad5d 100644
--- a/Classes/Core/Functional/Framework/Frontend/Renderer.php
+++ b/Classes/Core/Functional/Framework/Frontend/Renderer.php
@@ -38,7 +38,7 @@ class Renderer implements SingletonInterface
      * @param string $content
      * @param array|null $configuration
      */
-    public function parseValues($content, array $configuration = null)
+    public function parseValues($content, ?array $configuration = null)
     {
         if (empty($content)) {
             return;
@@ -82,7 +82,7 @@ public function parseValues($content, array $configuration = null)
      * @param string $content
      * @param array|null $configuration
      */
-    public function renderValues($content, array $configuration = null)
+    public function renderValues($content, ?array $configuration = null)
     {
         if (empty($configuration['values.'])) {
             return;
@@ -110,7 +110,7 @@ public function addSection(array $section, $as = null)
      * @param array|null $configuration
      * @return string
      */
-    public function renderSections($content, array $configuration = null)
+    public function renderSections($content, ?array $configuration = null)
     {
         return json_encode($this->sections);
     }
diff --git a/Classes/Core/Functional/Framework/Frontend/RequestBootstrap.php b/Classes/Core/Functional/Framework/Frontend/RequestBootstrap.php
index f8b2a40d..e5289c3c 100644
--- a/Classes/Core/Functional/Framework/Frontend/RequestBootstrap.php
+++ b/Classes/Core/Functional/Framework/Frontend/RequestBootstrap.php
@@ -58,7 +58,7 @@ class RequestBootstrap
      * @param string $documentRoot
      * @param array|null $this->requestArguments
      */
-    public function __construct(string $documentRoot, array $requestArguments = null)
+    public function __construct(string $documentRoot, ?array $requestArguments = null)
     {
         $this->documentRoot = $documentRoot;
         $this->requestArguments = $requestArguments;
diff --git a/Classes/Core/Functional/Framework/Frontend/ResponseContent.php b/Classes/Core/Functional/Framework/Frontend/ResponseContent.php
index 92d7b264..ccba829c 100644
--- a/Classes/Core/Functional/Framework/Frontend/ResponseContent.php
+++ b/Classes/Core/Functional/Framework/Frontend/ResponseContent.php
@@ -43,7 +43,7 @@ class ResponseContent
      */
     protected $scope = [];
 
-    public static function fromString(string $data, ResponseContent $target = null): ResponseContent
+    public static function fromString(string $data, ?ResponseContent $target = null): ResponseContent
     {
         $target = $target ?? new static();
         $content = json_decode($data, true);
@@ -65,7 +65,7 @@ public static function fromString(string $data, ResponseContent $target = null):
     /**
      * @param Response $response (deprecated)
      */
-    final public function __construct(Response $response = null)
+    final public function __construct(?Response $response = null)
     {
         if ($response instanceof Response) {
             static::fromString($response->getContent(), $this);
diff --git a/Classes/Core/Functional/FunctionalTestCase.php b/Classes/Core/Functional/FunctionalTestCase.php
index fc35fbbb..23af19dc 100644
--- a/Classes/Core/Functional/FunctionalTestCase.php
+++ b/Classes/Core/Functional/FunctionalTestCase.php
@@ -1117,7 +1117,7 @@ protected function addTypoScriptToTemplateRecord(int $pageId, string $typoScript
      */
     protected function executeFrontendSubRequest(
         InternalRequest $request,
-        InternalRequestContext $context = null,
+        ?InternalRequestContext $context = null,
         bool $followRedirects = false
     ): InternalResponse {
         if ($context === null) {
@@ -1285,7 +1285,7 @@ private function retrieveFrontendSubRequestResult(
      */
     protected function executeFrontendRequest(
         InternalRequest $request,
-        InternalRequestContext $context = null,
+        ?InternalRequestContext $context = null,
         bool $followRedirects = false
     ): InternalResponse {
         if ($context === null) {