diff --git a/src/Tool/ApiCredentials.php b/src/Tool/ApiCredentials.php
index abfe5f9..8fa04cc 100644
--- a/src/Tool/ApiCredentials.php
+++ b/src/Tool/ApiCredentials.php
@@ -40,7 +40,7 @@ final class ApiCredentials
     /**
      * @var string
      */
-    private $refreshToken;
+    private $refreshToken = '';
 
     /**
      * @param string $authKey
@@ -121,4 +121,4 @@ public function getRefreshToken()
     {
         return $this->refreshToken;
     }
-}
\ No newline at end of file
+}