diff --git a/Security/Authenticator/RefreshTokenAuthenticator.php b/Security/Authenticator/RefreshTokenAuthenticator.php
index fa9e128e..eeb6bf2f 100644
--- a/Security/Authenticator/RefreshTokenAuthenticator.php
+++ b/Security/Authenticator/RefreshTokenAuthenticator.php
@@ -146,7 +146,7 @@ public function onAuthenticationFailure(Request $request, AuthenticationExceptio
     /**
      * @return Response
      */
-    public function start(Request $request, AuthenticationException $authException = null)
+    public function start(Request $request, ?AuthenticationException $authException = null)
     {
         $data = [
             // you might translate this message