diff --git a/src/Rest/OAuthRestClientException.php b/src/Rest/OAuthRestClientException.php
index 5cb305e..cf14b54 100644
--- a/src/Rest/OAuthRestClientException.php
+++ b/src/Rest/OAuthRestClientException.php
@@ -10,7 +10,7 @@ public static function unableToLoadAccessToken(?string $message = null)
         $errorMessage = 'Unable to load access token';
         
         if ($message) {
-            $errorMessage . ': ' . $message;
+            $errorMessage = $errorMessage . ': ' . $message;
         }
 
         return new self($errorMessage);