diff --git a/apps/updatenotification/lib/BackgroundJob/UpdateAvailableNotifications.php b/apps/updatenotification/lib/BackgroundJob/UpdateAvailableNotifications.php
index f561f38fdc091..901a36429a25c 100644
--- a/apps/updatenotification/lib/BackgroundJob/UpdateAvailableNotifications.php
+++ b/apps/updatenotification/lib/BackgroundJob/UpdateAvailableNotifications.php
@@ -121,7 +121,7 @@ protected function sendErrorNotifications($numDays) {
 				->setSubject('connection_error', ['days' => $numDays]);
 
 			foreach ($this->getUsersToNotify() as $uid) {
-				$notification->setUser($uid);
+				$notification->setUser((string) $uid);
 				$this->notificationManager->notify($notification);
 			}
 		} catch (\InvalidArgumentException $e) {
@@ -189,7 +189,7 @@ protected function createNotifications($app, $version, $visibleVersion = '') {
 			}
 
 			foreach ($this->getUsersToNotify() as $uid) {
-				$notification->setUser($uid);
+				$notification->setUser((string) $uid);
 				$this->notificationManager->notify($notification);
 			}
 		} catch (\InvalidArgumentException $e) {