diff --git a/lib/OAuth2/OAuth2.php b/lib/OAuth2/OAuth2.php
index 898e681..5505319 100644
--- a/lib/OAuth2/OAuth2.php
+++ b/lib/OAuth2/OAuth2.php
@@ -904,7 +904,7 @@ protected function grantAccessTokenUserCredentials(IOAuth2Client $client, array
         $stored = $this->storage->checkUserCredentials($client, $input["username"], $input["password"]);
 
         if ($stored === false) {
-            throw new OAuth2ServerException(self::HTTP_BAD_REQUEST, self::ERROR_INVALID_GRANT);
+            throw new OAuth2ServerException(self::HTTP_BAD_REQUEST, self::ERROR_INVALID_GRANT, "Invalid username and password combination");
         }
 
         return $stored;