diff --git a/src/Plugin/CookiePlugin.php b/src/Plugin/CookiePlugin.php
index 6a8942a..23c8593 100644
--- a/src/Plugin/CookiePlugin.php
+++ b/src/Plugin/CookiePlugin.php
@@ -164,7 +164,7 @@ private function createCookie(RequestInterface $request, string $setCookieHeader
      *
      * @param string $part A single cookie value in format key=value
      *
-     * @return array{0:string, 1:?string}
+     * @return array{0:string, 1:string|null}
      */
     private function createValueKey(string $part): array
     {