diff --git a/CHANGELOG.md b/CHANGELOG.md
index f592f37..94b03a1 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -10,6 +10,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 ### Fixed
 
+- Fix undefined index
 - Prevent deletion of actors during escalation.
 - Fix Uncaught ReferenceError: __s is not defined
 
diff --git a/inc/ticket.class.php b/inc/ticket.class.php
index 5e92d04..28087e1 100644
--- a/inc/ticket.class.php
+++ b/inc/ticket.class.php
@@ -903,7 +903,11 @@ public static function linkedTickets(CommonDBTM $ticket, $status = CommonITILObj
         $linkedTicket = new Ticket();
 
         foreach ($tickets as $data) {
-            if ($data['link'] !== Ticket_Ticket::LINK_TO || !$linkedTicket->can($data['tickets_id'], UPDATE)) {
+            $ticket_id_missing = empty($data['tickets_id']);
+            $link_invalid = empty($data['link']) || $data['link'] !== Ticket_Ticket::LINK_TO;
+            $no_permission = !$linkedTicket->can($data['tickets_id'], UPDATE);
+
+            if ($ticket_id_missing || $link_invalid || $no_permission) {
                 continue;
             }