diff --git a/src/TrailingSlash.php b/src/TrailingSlash.php
index db5caa1..f247e31 100644
--- a/src/TrailingSlash.php
+++ b/src/TrailingSlash.php
@@ -33,7 +33,7 @@ public function __construct(bool $trailingSlash = false)
     /**
      * Whether returns a 301 response to the new path.
      */
-    public function redirect(ResponseFactoryInterface $responseFactory = null): self
+    public function redirect(?ResponseFactoryInterface $responseFactory = null): self
     {
         $this->responseFactory = $responseFactory ?: Factory::getResponseFactory();