diff --git a/src/ResponseHeaders.php b/src/ResponseHeaders.php
index a7586d5..04532ce 100644
--- a/src/ResponseHeaders.php
+++ b/src/ResponseHeaders.php
@@ -42,7 +42,7 @@ public function set(string $key, string $value): void
      * @param string|null $default
      * @return THeaderValue|null
      */
-    public function get(string $key, string $default = null): ?string
+    public function get(string $key, ?string $default = null): ?string
     {
         return $this->headers[$key] ?? $default;
     }