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/MiddlewarePipeline.php b/src/MiddlewarePipeline.php
index f5dda09..82365fb 100644
--- a/src/MiddlewarePipeline.php
+++ b/src/MiddlewarePipeline.php
@@ -23,8 +23,9 @@ final class MiddlewarePipeline implements MiddlewarePipelineInterface
 
     /**
      * {@inheritDoc}
+     * @psalm-suppress RiskyTruthyFalsyComparison
      */
-    public function pipe(MiddlewareInterface $middleware, string $pathPrefix = null): void
+    public function pipe(MiddlewareInterface $middleware, ?string $pathPrefix = null): void
     {
         $this->pipeline[] = (!$pathPrefix || $pathPrefix === '/') ? $middleware : $this->path($pathPrefix, $middleware);
     }
diff --git a/src/MiddlewarePipelineInterface.php b/src/MiddlewarePipelineInterface.php
index 6f44aa9..3d5c00a 100644
--- a/src/MiddlewarePipelineInterface.php
+++ b/src/MiddlewarePipelineInterface.php
@@ -51,5 +51,5 @@ interface MiddlewarePipelineInterface extends MiddlewareInterface, RequestHandle
      * @param MiddlewareInterface $middleware
      * @param string|null $pathPrefix path prefix from the root to which the middleware is attached.
      */
-    public function pipe(MiddlewareInterface $middleware, string $pathPrefix = null): void;
+    public function pipe(MiddlewareInterface $middleware, ?string $pathPrefix = null): void;
 }
diff --git a/src/MiddlewareResolver.php b/src/MiddlewareResolver.php
index 857bc0f..ec2d181 100644
--- a/src/MiddlewareResolver.php
+++ b/src/MiddlewareResolver.php
@@ -26,7 +26,7 @@ final class MiddlewareResolver implements MiddlewareResolverInterface
     /**
      * @param ContainerInterface|null $container
      */
-    public function __construct(ContainerInterface $container = null)
+    public function __construct(?ContainerInterface $container = null)
     {
         $this->container = $container;
     }
diff --git a/src/ServerRequestRunner.php b/src/ServerRequestRunner.php
index 31cb95f..ba9e83a 100644
--- a/src/ServerRequestRunner.php
+++ b/src/ServerRequestRunner.php
@@ -33,7 +33,7 @@ final class ServerRequestRunner
      * @param MiddlewarePipelineInterface|null $pipeline
      * @param EmitterInterface|null $emitter
      */
-    public function __construct(MiddlewarePipelineInterface $pipeline = null, EmitterInterface $emitter = null)
+    public function __construct(?MiddlewarePipelineInterface $pipeline = null, ?EmitterInterface $emitter = null)
     {
         $this->pipeline = $pipeline ?? new MiddlewarePipeline();
         $this->emitter = $emitter ?? new SapiEmitter();
@@ -44,7 +44,7 @@ public function __construct(MiddlewarePipelineInterface $pipeline = null, Emitte
      * @param RequestHandlerInterface|null $defaultHandler
      * @psalm-suppress RedundantCast
      */
-    public function run(ServerRequestInterface $request, RequestHandlerInterface $defaultHandler = null): void
+    public function run(ServerRequestInterface $request, ?RequestHandlerInterface $defaultHandler = null): void
     {
         $response = ($defaultHandler === null)
             ? $this->pipeline->handle($request)