diff --git a/src/Application.php b/src/Application.php
index cd16517..7728ccf 100644
--- a/src/Application.php
+++ b/src/Application.php
@@ -52,7 +52,7 @@ public function __construct(
         EmitterInterface $emitter,
         MiddlewarePipelineInterface $pipeline,
         MiddlewareResolverInterface $resolver,
-        RequestHandlerInterface $default = null
+        ?RequestHandlerInterface $default = null
     ) {
         $this->router = $router;
         $this->emitter = $emitter;
diff --git a/src/Exception/BadRequestHttpException.php b/src/Exception/BadRequestHttpException.php
index e04d998..8ad8ed8 100644
--- a/src/Exception/BadRequestHttpException.php
+++ b/src/Exception/BadRequestHttpException.php
@@ -13,7 +13,7 @@ class BadRequestHttpException extends HttpException
      * @param string|null $reasonPhrase
      * @param Throwable|null $previous
      */
-    public function __construct(string $reasonPhrase = null, Throwable $previous = null)
+    public function __construct(?string $reasonPhrase = null, ?Throwable $previous = null)
     {
         parent::__construct(ErrorResponseGeneratorInterface::STATUS_BAD_REQUEST, $reasonPhrase, $previous);
     }
diff --git a/src/Exception/ForbiddenHttpException.php b/src/Exception/ForbiddenHttpException.php
index 3e14b9c..ea9ab8e 100644
--- a/src/Exception/ForbiddenHttpException.php
+++ b/src/Exception/ForbiddenHttpException.php
@@ -13,7 +13,7 @@ class ForbiddenHttpException extends HttpException
      * @param string|null $reasonPhrase
      * @param Throwable|null $previous
      */
-    public function __construct(string $reasonPhrase = null, Throwable $previous = null)
+    public function __construct(?string $reasonPhrase = null, ?Throwable $previous = null)
     {
         parent::__construct(ErrorResponseGeneratorInterface::STATUS_FORBIDDEN, $reasonPhrase, $previous);
     }
diff --git a/src/Exception/HttpException.php b/src/Exception/HttpException.php
index faca005..eae4aa3 100644
--- a/src/Exception/HttpException.php
+++ b/src/Exception/HttpException.php
@@ -25,7 +25,7 @@ class HttpException extends Exception
      * @param string|null $reasonPhrase
      * @param Throwable|null $previous
      */
-    public function __construct(int $statusCode, string $reasonPhrase = null, Throwable $previous = null)
+    public function __construct(int $statusCode, ?string $reasonPhrase = null, ?Throwable $previous = null)
     {
         $reasonPhrase ??= '';
 
diff --git a/src/Exception/InternalServerErrorHttpException.php b/src/Exception/InternalServerErrorHttpException.php
index 7e73f1b..7afc952 100644
--- a/src/Exception/InternalServerErrorHttpException.php
+++ b/src/Exception/InternalServerErrorHttpException.php
@@ -13,7 +13,7 @@ class InternalServerErrorHttpException extends HttpException
      * @param string|null $reasonPhrase
      * @param Throwable|null $previous
      */
-    public function __construct(string $reasonPhrase = null, Throwable $previous = null)
+    public function __construct(?string $reasonPhrase = null, ?Throwable $previous = null)
     {
         parent::__construct(ErrorResponseGeneratorInterface::STATUS_INTERNAL_SERVER_ERROR, $reasonPhrase, $previous);
     }
diff --git a/src/Exception/MethodNotAllowedHttpException.php b/src/Exception/MethodNotAllowedHttpException.php
index d29d330..c439f83 100644
--- a/src/Exception/MethodNotAllowedHttpException.php
+++ b/src/Exception/MethodNotAllowedHttpException.php
@@ -13,7 +13,7 @@ class MethodNotAllowedHttpException extends HttpException
      * @param string|null $reasonPhrase
      * @param Throwable|null $previous
      */
-    public function __construct(string $reasonPhrase = null, Throwable $previous = null)
+    public function __construct(?string $reasonPhrase = null, ?Throwable $previous = null)
     {
         parent::__construct(ErrorResponseGeneratorInterface::STATUS_METHOD_NOT_ALLOWED, $reasonPhrase, $previous);
     }
diff --git a/src/Exception/NotFoundHttpException.php b/src/Exception/NotFoundHttpException.php
index f97e8bb..28e9a74 100644
--- a/src/Exception/NotFoundHttpException.php
+++ b/src/Exception/NotFoundHttpException.php
@@ -13,7 +13,7 @@ class NotFoundHttpException extends HttpException
      * @param string|null $reasonPhrase
      * @param Throwable|null $previous
      */
-    public function __construct(string $reasonPhrase = null, Throwable $previous = null)
+    public function __construct(?string $reasonPhrase = null, ?Throwable $previous = null)
     {
         parent::__construct(ErrorResponseGeneratorInterface::STATUS_NOT_FOUND, $reasonPhrase, $previous);
     }
diff --git a/src/Exception/NotImplementedHttpException.php b/src/Exception/NotImplementedHttpException.php
index 3af9cad..5554c14 100644
--- a/src/Exception/NotImplementedHttpException.php
+++ b/src/Exception/NotImplementedHttpException.php
@@ -13,7 +13,7 @@ class NotImplementedHttpException extends HttpException
      * @param string|null $reasonPhrase
      * @param Throwable|null $previous
      */
-    public function __construct(string $reasonPhrase = null, Throwable $previous = null)
+    public function __construct(?string $reasonPhrase = null, ?Throwable $previous = null)
     {
         parent::__construct(ErrorResponseGeneratorInterface::STATUS_NOT_IMPLEMENTED, $reasonPhrase, $previous);
     }
diff --git a/src/Exception/UnauthorizedHttpException.php b/src/Exception/UnauthorizedHttpException.php
index 75fe263..739165f 100644
--- a/src/Exception/UnauthorizedHttpException.php
+++ b/src/Exception/UnauthorizedHttpException.php
@@ -13,7 +13,7 @@ class UnauthorizedHttpException extends HttpException
      * @param string|null $reasonPhrase
      * @param Throwable|null $previous
      */
-    public function __construct(string $reasonPhrase = null, Throwable $previous = null)
+    public function __construct(?string $reasonPhrase = null, ?Throwable $previous = null)
     {
         parent::__construct(ErrorResponseGeneratorInterface::STATUS_UNAUTHORIZED, $reasonPhrase, $previous);
     }
diff --git a/src/Response/CustomResponseFactory.php b/src/Response/CustomResponseFactory.php
index 92d94a7..6d710f9 100644
--- a/src/Response/CustomResponseFactory.php
+++ b/src/Response/CustomResponseFactory.php
@@ -31,7 +31,7 @@ final class CustomResponseFactory implements ResponseFactoryInterface
      * @param StreamInterface|string|resource $body
      * @param string $protocol
      */
-    public function __construct(array $headers = null, $body = 'php://temp', string $protocol = '1.1')
+    public function __construct(?array $headers = null, $body = 'php://temp', string $protocol = '1.1')
     {
         $this->headers = $headers ?? ['Content-Type' => 'text/html; charset=UTF-8'];
         $this->body = $body;