diff --git a/test/Transport/SendmailTest.php b/test/Transport/SendmailTest.php
index 6d531fa4..209dea80 100644
--- a/test/Transport/SendmailTest.php
+++ b/test/Transport/SendmailTest.php
@@ -27,6 +27,7 @@ class SendmailTest extends TestCase
     public $message;
     public $additional_headers;
     public $additional_parameters;
+    public $operating_system;
 
     public function setUp()
     {