diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index 7fa0ea1..420cb9b 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -39,6 +39,8 @@ jobs:
           - "8.0"
           - "8.1"
           - "8.2"
+          - "8.3"
+          - "8.4"
 
     steps:
       - name: Checkout.
diff --git a/.github/workflows/static.yml b/.github/workflows/static.yml
index 3e79b31..0d90f64 100644
--- a/.github/workflows/static.yml
+++ b/.github/workflows/static.yml
@@ -35,6 +35,7 @@ jobs:
           - "8.0"
           - "8.1"
           - "8.2"
+          - "8.3"
 
     steps:
       - name: Checkout.
diff --git a/src/ErrorHandler.php b/src/ErrorHandler.php
index 31923d6..c931f41 100644
--- a/src/ErrorHandler.php
+++ b/src/ErrorHandler.php
@@ -29,7 +29,7 @@ final class ErrorHandler implements RequestHandlerInterface
      */
     public function __construct(
         RequestHandlerInterface $handler,
-        ErrorResponseGeneratorInterface $responseGenerator = null
+        ?ErrorResponseGeneratorInterface $responseGenerator = null
     ) {
         $this->handler = $handler;
         $this->responseGenerator = $responseGenerator ?? new ErrorResponseGenerator();
diff --git a/src/ErrorHandlerMiddleware.php b/src/ErrorHandlerMiddleware.php
index ead9334..4fb95b9 100644
--- a/src/ErrorHandlerMiddleware.php
+++ b/src/ErrorHandlerMiddleware.php
@@ -17,7 +17,7 @@ final class ErrorHandlerMiddleware implements MiddlewareInterface
     /**
      * @param ErrorResponseGeneratorInterface|null $responseGenerator
      */
-    public function __construct(ErrorResponseGeneratorInterface $responseGenerator = null)
+    public function __construct(?ErrorResponseGeneratorInterface $responseGenerator = null)
     {
         $this->responseGenerator = $responseGenerator ?? new ErrorResponseGenerator();
     }