diff --git a/granttypes/RefreshToken.php b/granttypes/RefreshToken.php
index <HASH>..<HASH> 100644
--- a/granttypes/RefreshToken.php
+++ b/granttypes/RefreshToken.php
@@ -100,7 +100,7 @@ class RefreshToken extends BaseModel
             if (empty($this->refresh_token)) {
                 $this->errorServer('The request is missing "refresh_token" parameter');
             }
-            if (!$this->_refreshToken = RefreshToken::findOne(['refresh_token' => $this->refresh_token])) {
+            if (!$this->_refreshToken = \conquer\oauth2\models\RefreshToken::findOne(['refresh_token' => $this->refresh_token])) {
                 $this->errorServer('The Refresh Token is invalid');
             }
         }
Update RefreshToken.php

Add full namespace for RefreshToken. Else it reference to current class so findOne() method can't be found here.