diff --git a/lib/private/Notification/Notification.php b/lib/private/Notification/Notification.php
index ef7cdb5d44..ff3826047d 100644
--- a/lib/private/Notification/Notification.php
+++ b/lib/private/Notification/Notification.php
@@ -1,4 +1,5 @@
 <?php
+declare (strict_types = 1);
 /**
  * @copyright Copyright (c) 2016, ownCloud, Inc.
  *
@@ -133,7 +134,7 @@ class Notification implements INotification {
 	 * @throws \InvalidArgumentException if the app id is invalid
 	 * @since 8.2.0
 	 */
-	public function setApp($app) {
+	public function setApp(string $app) {
 		if (!is_string($app) || $app === '' || isset($app[32])) {
 			throw new \InvalidArgumentException('The given app name is invalid');
 		}
@@ -155,7 +156,7 @@ class Notification implements INotification {
 	 * @throws \InvalidArgumentException if the user id is invalid
 	 * @since 8.2.0
 	 */
-	public function setUser($user) {
+	public function setUser(string $user) {
 		if (!is_string($user) || $user === '' || isset($user[64])) {
 			throw new \InvalidArgumentException('The given user id is invalid');
 		}
diff --git a/lib/private/Repair/RemoveLinkShares.php b/lib/private/Repair/RemoveLinkShares.php
index dd020dae50..ed1be70533 100644
--- a/lib/private/Repair/RemoveLinkShares.php
+++ b/lib/private/Repair/RemoveLinkShares.php
@@ -196,7 +196,7 @@ class RemoveLinkShares implements IRepairStep {
 
 		$users = array_keys($this->userToNotify);
 		foreach ($users as $user) {
-			$notification->setUser($user);
+			$notification->setUser((string) $user);
 			$this->notificationManager->notify($notification);
 		}
 	}
diff --git a/lib/public/Notification/INotification.php b/lib/public/Notification/INotification.php
index 29f2a0e794..f71d0c53cc 100644
--- a/lib/public/Notification/INotification.php
+++ b/lib/public/Notification/INotification.php
@@ -1,4 +1,5 @@
 <?php
+declare (strict_types = 1);
 /**
  * @copyright Copyright (c) 2016, ownCloud, Inc.
  *
@@ -36,7 +37,7 @@ interface INotification {
 	 * @throws \InvalidArgumentException if the app id is invalid
 	 * @since 9.0.0
 	 */
-	public function setApp($app);
+	public function setApp(string $app);
 
 	/**
 	 * @return string
@@ -50,7 +51,7 @@ interface INotification {
 	 * @throws \InvalidArgumentException if the user id is invalid
 	 * @since 9.0.0
 	 */
-	public function setUser($user);
+	public function setUser(string $user);
 
 	/**
 	 * @return string