diff --git a/src/Request.php b/src/Request.php
index c4630ca..af3fa3f 100644
--- a/src/Request.php
+++ b/src/Request.php
@@ -85,6 +85,10 @@ protected static function ticket($verifySSL, $verifyHost)
       if (!$response) {
         throw new ProxmoxException('Request params empty');
       }
+      
+      if ($response->data === NULL) {
+        throw new ProxmoxException('Invalid credentials');
+      }
 
       // set header
       self::$Client->setHeader('CSRFPreventionToken', $response->data->CSRFPreventionToken);