diff --git a/src/Response.php b/src/Response.php
index 39d68cc5..d50e8d92 100644
--- a/src/Response.php
+++ b/src/Response.php
@@ -184,7 +184,7 @@ public function removeHeader(string $name): void
      *
      * @param int $status 100 - 599
      */
-    public function setStatus(int $status, string $reason = null): void
+    public function setStatus(int $status, ?string $reason = null): void
     {
         parent::setStatus($this->validateStatusCode($status), $reason);