diff --git a/src/Grant/RefreshTokenGrant.php b/src/Grant/RefreshTokenGrant.php
index 42cef93d4..3a9516097 100644
--- a/src/Grant/RefreshTokenGrant.php
+++ b/src/Grant/RefreshTokenGrant.php
@@ -78,7 +78,10 @@ public function respondToAccessTokenRequest(
         }
 
         // Issue and persist new access token
-        $accessToken = $this->issueAccessToken($accessTokenTTL, $client, (string)$oldRefreshToken['user_id'], $scopes);
+        $userId = $oldRefreshToken['user_id'];
+        if (is_int($userId))
+            $userId = (string)$userId;
+        $accessToken = $this->issueAccessToken($accessTokenTTL, $client, $userId, $scopes);
         $this->getEmitter()->emit(new RequestAccessTokenEvent(RequestEvent::ACCESS_TOKEN_ISSUED, $request, $accessToken));
         $responseType->setAccessToken($accessToken);