diff --git a/.github/workflows/tests.yml b/.github/workflows/tests.yml
index f93a781..a45eaf0 100644
--- a/.github/workflows/tests.yml
+++ b/.github/workflows/tests.yml
@@ -11,7 +11,7 @@ jobs:
 
     strategy:
       matrix:
-        php: ['5.5', '5.6', '7.0', '7.1', '7.2', '7.3', '7.4', '8.0', '8.1', '8.2']
+        php: ['5.5', '5.6', '7.0', '7.1', '7.2', '7.3', '7.4', '8.0', '8.1', '8.2', '8.3', '8.4']
 
     steps:
       - name: Checkout Code
diff --git a/src/Whoops/Handler/PrettyPageHandler.php b/src/Whoops/Handler/PrettyPageHandler.php
index 167407e..b739ac0 100644
--- a/src/Whoops/Handler/PrettyPageHandler.php
+++ b/src/Whoops/Handler/PrettyPageHandler.php
@@ -383,7 +383,7 @@ class PrettyPageHandler extends Handler
             throw new InvalidArgumentException('Expecting callback argument to be callable');
         }
 
-        $this->extraTables[$label] = function (\Whoops\Inspector\InspectorInterface $inspector = null) use ($callback) {
+        $this->extraTables[$label] = function (?\Whoops\Inspector\InspectorInterface $inspector = null) use ($callback) {
             try {
                 $result = call_user_func($callback, $inspector);
 
diff --git a/src/Whoops/Run.php b/src/Whoops/Run.php
index 0862768..7be63af 100644
--- a/src/Whoops/Run.php
+++ b/src/Whoops/Run.php
@@ -81,7 +81,7 @@ final class Run implements RunInterface
      */
     private $frameFilters = [];
 
-    public function __construct(SystemFacade $system = null)
+    public function __construct(?SystemFacade $system = null)
     {
         $this->system = $system ?: new SystemFacade;
         $this->inspectorFactory = new InspectorFactory();
diff --git a/src/Whoops/Util/TemplateHelper.php b/src/Whoops/Util/TemplateHelper.php
index 8e4df32..5612c0b 100644
--- a/src/Whoops/Util/TemplateHelper.php
+++ b/src/Whoops/Util/TemplateHelper.php
@@ -233,7 +233,7 @@ class TemplateHelper
      *
      * @param string $template
      */
-    public function render($template, array $additionalVariables = null)
+    public function render($template, ?array $additionalVariables = null)
     {
         $variables = $this->getVariables();