diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 3c7beb2..f03bd5f 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -27,6 +27,15 @@ jobs:
           - operating-system: 'ubuntu-latest'
             php-version: '8.1'
 
+          - operating-system: 'ubuntu-latest'
+            php-version: '8.2'
+
+          - operating-system: 'ubuntu-latest'
+            php-version: '8.3'
+
+          - operating-system: 'ubuntu-latest'
+            php-version: '8.4'
+
           - operating-system: 'windows-latest'
             php-version: '8.1'
             job-description: 'on Windows'
diff --git a/src/AsyncTestCase.php b/src/AsyncTestCase.php
index bd9efd5..e16a11c 100644
--- a/src/AsyncTestCase.php
+++ b/src/AsyncTestCase.php
@@ -241,7 +241,7 @@ final protected function setTimeout(int $timeout)
      *
      * @return callable|MockObject Mock object having only an __invoke method.
      */
-    final protected function createCallback(int $invocationCount, callable $returnCallback = null): callable
+    final protected function createCallback(int $invocationCount, ?callable $returnCallback = null): callable
     {
         $mock = $this->createMock(CallbackStub::class);
         $invocationMocker = $mock->expects($this->exactly($invocationCount))