diff --git a/modules/Vtiger/actions/MentionedUsers.php b/modules/Vtiger/actions/MentionedUsers.php
index 4f71393f7fff004afc97d9580dc76c3fd3f782d7..d6bd2cba374f675d978cc4ec6e32e840f160d38c 100644
--- a/modules/Vtiger/actions/MentionedUsers.php
+++ b/modules/Vtiger/actions/MentionedUsers.php
@@ -38,8 +38,10 @@ class Vtiger_MentionedUsers_Action extends Vtiger_Action_Controller {
             unset($mentionedUsers[$key]);
         }
         
-        $usersString = '@'.implode(' @', $mentionedUsers);
-        $mentionedUsersData['usersString'] = $usersString.' ';
+        if(!empty($mentionedUsers)){
+            $usersString = '@'.implode(' @', $mentionedUsers);
+            $mentionedUsersData['usersString'] = $usersString.' ';
+        }
         
         $response = new Vtiger_Response();
         $response->setResult($mentionedUsersData);