diff --git a/plugins/kolab_addressbook/lib/rcube_kolab_contacts.php b/plugins/kolab_addressbook/lib/rcube_kolab_contacts.php
index 4baef45f..365e2510 100644
--- a/plugins/kolab_addressbook/lib/rcube_kolab_contacts.php
+++ b/plugins/kolab_addressbook/lib/rcube_kolab_contacts.php
@@ -933,7 +933,7 @@ class rcube_kolab_contacts extends rcube_addressbook
      *
      * @return boolean True if input is valid, False if not.
      */
-    public function validate($save_data)
+    public function validate(&$save_data)
     {
         // validate e-mail addresses
         $valid = parent::validate($save_data);