diff --git a/lib/Notification/Notifier.php b/lib/Notification/Notifier.php
index 2e0341c82d8..823785e422b 100644
--- a/lib/Notification/Notifier.php
+++ b/lib/Notification/Notifier.php
@@ -580,7 +580,7 @@ protected function parseChatMessage(INotification $notification, Room $room, Par
 
 		$richSubjectCall = [
 			'type' => 'call',
-			'id' => $room->getId(),
+			'id' => (string)$room->getId(),
 			'name' => $room->getDisplayName($notification->getUser()),
 			'call-type' => $this->getRoomType($room),
 			'icon-url' => $this->avatarService->getAvatarUrl($room),