diff --git a/src/Grant/AuthCodeGrant.php b/src/Grant/AuthCodeGrant.php
index c7c7e8c9..08c78c8a 100644
--- a/src/Grant/AuthCodeGrant.php
+++ b/src/Grant/AuthCodeGrant.php
@@ -199,7 +199,7 @@ class AuthCodeGrant extends AbstractAuthorizeGrant
     /**
      * Fetch the client_id parameter from the query string.
      *
-     * @return string
+     * @return string|null
      * @throws OAuthServerException
      */
     protected function getClientIdFromRequest($request)