diff --git a/src/Grant/AbstractGrant.php b/src/Grant/AbstractGrant.php
index 90c75d51..45269ac7 100644
--- a/src/Grant/AbstractGrant.php
+++ b/src/Grant/AbstractGrant.php
@@ -478,7 +478,7 @@ abstract class AbstractGrant implements GrantTypeInterface
     {
         $refreshToken = $this->refreshTokenRepository->getNewRefreshToken();
         if ($refreshToken === null) {
-            return;
+            return null;
         }
 
         $refreshToken->setExpiryDateTime((new DateTime())->add($this->refreshTokenTTL));