diff --git a/apps/comments/tests/Unit/Controller/NotificationsTest.php b/apps/comments/tests/Unit/Controller/NotificationsTest.php
index 9897027b82..c5209b9f9d 100644
--- a/apps/comments/tests/Unit/Controller/NotificationsTest.php
+++ b/apps/comments/tests/Unit/Controller/NotificationsTest.php
@@ -125,6 +125,7 @@ class NotificationsTest extends TestCase {
 
 		$file = $this->createMock(Node::class);
 		$folder = $this->createMock(Folder::class);
+		$user = $this->createMock(IUser::class);
 
 		$this->rootFolder->expects($this->once())
 			->method('getUserFolder')
@@ -136,7 +137,11 @@ class NotificationsTest extends TestCase {
 
 		$this->session->expects($this->once())
 			->method('getUser')
-			->willReturn($this->createMock(IUser::class));
+			->willReturn($user);
+
+		$user->expects($this->any())
+			->method('getUID')
+			->willReturn('user');
 
 		$notification = $this->createMock(INotification::class);
 		$notification->expects($this->any())
@@ -163,9 +168,15 @@ class NotificationsTest extends TestCase {
 		$this->rootFolder->expects($this->never())
 			->method('getUserFolder');
 
+		$user = $this->createMock(IUser::class);
+
 		$this->session->expects($this->once())
 			->method('getUser')
-			->willReturn($this->createMock(IUser::class));
+			->willReturn($user);
+
+		$user->expects($this->any())
+			->method('getUID')
+			->willReturn('user');
 
 		$this->notificationManager->expects($this->never())
 			->method('createNotification');
@@ -197,9 +208,15 @@ class NotificationsTest extends TestCase {
 			->method('getById')
 			->willReturn([]);
 
+		$user = $this->createMock(IUser::class);
+
 		$this->session->expects($this->once())
 			->method('getUser')
-			->willReturn($this->createMock(IUser::class));
+			->willReturn($user);
+
+		$user->expects($this->any())
+			->method('getUID')
+			->willReturn('user');
 
 		$notification = $this->createMock(INotification::class);
 		$notification->expects($this->any())
diff --git a/lib/private/Notification/Notification.php b/lib/private/Notification/Notification.php
index ff3826047d..47f415d15d 100644
--- a/lib/private/Notification/Notification.php
+++ b/lib/private/Notification/Notification.php
@@ -135,7 +135,7 @@ class Notification implements INotification {
 	 * @since 8.2.0
 	 */
 	public function setApp(string $app) {
-		if (!is_string($app) || $app === '' || isset($app[32])) {
+		if (trim($app) === '' || isset($app[32])) {
 			throw new \InvalidArgumentException('The given app name is invalid');
 		}
 		$this->app = $app;
@@ -157,7 +157,7 @@ class Notification implements INotification {
 	 * @since 8.2.0
 	 */
 	public function setUser(string $user) {
-		if (!is_string($user) || $user === '' || isset($user[64])) {
+		if (trim($user) === '' || isset($user[64])) {
 			throw new \InvalidArgumentException('The given user id is invalid');
 		}
 		$this->user = $user;
@@ -201,8 +201,8 @@ class Notification implements INotification {
 	 * @throws \InvalidArgumentException if the object type or id is invalid
 	 * @since 8.2.0 - 9.0.0: Type of $id changed to string
 	 */
-	public function setObject($type, $id) {
-		if (!is_string($type) || $type === '' || isset($type[64])) {
+	public function setObject(string $type, $id) {
+		if (trim($type) === '' || isset($type[64])) {
 			throw new \InvalidArgumentException('The given object type is invalid');
 		}
 		$this->objectType = $type;
@@ -237,8 +237,8 @@ class Notification implements INotification {
 	 * @throws \InvalidArgumentException if the subject or parameters are invalid
 	 * @since 8.2.0
 	 */
-	public function setSubject($subject, array $parameters = []) {
-		if (!is_string($subject) || $subject === '' || isset($subject[64])) {
+	public function setSubject(string $subject, array $parameters = []) {
+		if (trim($subject) === '' || isset($subject[64])) {
 			throw new \InvalidArgumentException('The given subject is invalid');
 		}
 
@@ -270,8 +270,8 @@ class Notification implements INotification {
 	 * @throws \InvalidArgumentException if the subject is invalid
 	 * @since 8.2.0
 	 */
-	public function setParsedSubject($subject) {
-		if (!is_string($subject) || $subject === '') {
+	public function setParsedSubject(string $subject) {
+		if (trim($subject) === '') {
 			throw new \InvalidArgumentException('The given parsed subject is invalid');
 		}
 		$this->subjectParsed = $subject;
@@ -293,8 +293,8 @@ class Notification implements INotification {
 	 * @throws \InvalidArgumentException if the subject or parameters are invalid
 	 * @since 11.0.0
 	 */
-	public function setRichSubject($subject, array $parameters = []) {
-		if (!is_string($subject) || $subject === '') {
+	public function setRichSubject(string $subject, array $parameters = []) {
+		if (trim($subject) === '') {
 			throw new \InvalidArgumentException('The given parsed subject is invalid');
 		}
 
@@ -327,8 +327,8 @@ class Notification implements INotification {
 	 * @throws \InvalidArgumentException if the message or parameters are invalid
 	 * @since 8.2.0
 	 */
-	public function setMessage($message, array $parameters = []) {
-		if (!is_string($message) || $message === '' || isset($message[64])) {
+	public function setMessage(string $message, array $parameters = []) {
+		if (trim($message) === '' || isset($message[64])) {
 			throw new \InvalidArgumentException('The given message is invalid');
 		}
 
@@ -360,8 +360,8 @@ class Notification implements INotification {
 	 * @throws \InvalidArgumentException if the message is invalid
 	 * @since 8.2.0
 	 */
-	public function setParsedMessage($message) {
-		if (!is_string($message) || $message === '') {
+	public function setParsedMessage(string $message) {
+		if (trim($message) === '') {
 			throw new \InvalidArgumentException('The given parsed message is invalid');
 		}
 		$this->messageParsed = $message;
@@ -383,8 +383,8 @@ class Notification implements INotification {
 	 * @throws \InvalidArgumentException if the message or parameters are invalid
 	 * @since 11.0.0
 	 */
-	public function setRichMessage($message, array $parameters = []) {
-		if (!is_string($message) || $message === '') {
+	public function setRichMessage(string $message, array $parameters = []) {
+		if (trim($message) === '') {
 			throw new \InvalidArgumentException('The given parsed message is invalid');
 		}
 
@@ -416,8 +416,8 @@ class Notification implements INotification {
 	 * @throws \InvalidArgumentException if the link is invalid
 	 * @since 8.2.0
 	 */
-	public function setLink($link) {
-		if (!is_string($link) || $link === '' || isset($link[4000])) {
+	public function setLink(string $link) {
+		if (trim($link) === '' || isset($link[4000])) {
 			throw new \InvalidArgumentException('The given link is invalid');
 		}
 		$this->link = $link;
@@ -438,8 +438,8 @@ class Notification implements INotification {
 	 * @throws \InvalidArgumentException if the icon is invalid
 	 * @since 11.0.0
 	 */
-	public function setIcon($icon) {
-		if (!is_string($icon) || $icon === '' || isset($icon[4000])) {
+	public function setIcon(string $icon) {
+		if (trim($icon) === '' || isset($icon[4000])) {
 			throw new \InvalidArgumentException('The given icon is invalid');
 		}
 		$this->icon = $icon;
diff --git a/lib/public/Notification/INotification.php b/lib/public/Notification/INotification.php
index f71d0c53cc..f7400e4778 100644
--- a/lib/public/Notification/INotification.php
+++ b/lib/public/Notification/INotification.php
@@ -80,7 +80,7 @@ interface INotification {
 	 * @throws \InvalidArgumentException if the object type or id is invalid
 	 * @since 9.0.0
 	 */
-	public function setObject($type, $id);
+	public function setObject(string $type, $id);
 
 	/**
 	 * @return string
@@ -101,7 +101,7 @@ interface INotification {
 	 * @throws \InvalidArgumentException if the subject or parameters are invalid
 	 * @since 9.0.0
 	 */
-	public function setSubject($subject, array $parameters = []);
+	public function setSubject(string $subject, array $parameters = []);
 
 	/**
 	 * @return string
@@ -132,7 +132,7 @@ interface INotification {
 	 * @throws \InvalidArgumentException if the subject is invalid
 	 * @since 9.0.0
 	 */
-	public function setParsedSubject($subject);
+	public function setParsedSubject(string $subject);
 
 	/**
 	 * @return string
@@ -157,7 +157,7 @@ interface INotification {
 	 * @throws \InvalidArgumentException if the subject or parameters are invalid
 	 * @since 11.0.0
 	 */
-	public function setRichSubject($subject, array $parameters = []);
+	public function setRichSubject(string $subject, array $parameters = []);
 
 	/**
 	 * @return string
@@ -178,7 +178,7 @@ interface INotification {
 	 * @throws \InvalidArgumentException if the message or parameters are invalid
 	 * @since 9.0.0
 	 */
-	public function setMessage($message, array $parameters = []);
+	public function setMessage(string $message, array $parameters = []);
 
 	/**
 	 * @return string
@@ -209,7 +209,7 @@ interface INotification {
 	 * @throws \InvalidArgumentException if the message is invalid
 	 * @since 9.0.0
 	 */
-	public function setParsedMessage($message);
+	public function setParsedMessage(string $message);
 
 	/**
 	 * @return string
@@ -234,7 +234,7 @@ interface INotification {
 	 * @throws \InvalidArgumentException if the message or parameters are invalid
 	 * @since 11.0.0
 	 */
-	public function setRichMessage($message, array $parameters = []);
+	public function setRichMessage(string $message, array $parameters = []);
 
 	/**
 	 * @return string
@@ -254,7 +254,7 @@ interface INotification {
 	 * @throws \InvalidArgumentException if the link is invalid
 	 * @since 9.0.0
 	 */
-	public function setLink($link);
+	public function setLink(string $link);
 
 	/**
 	 * @return string
@@ -268,7 +268,7 @@ interface INotification {
 	 * @throws \InvalidArgumentException if the icon is invalid
 	 * @since 11.0.0
 	 */
-	public function setIcon($icon);
+	public function setIcon(string $icon);
 
 	/**
 	 * @return string
diff --git a/tests/lib/Notification/NotificationTest.php b/tests/lib/Notification/NotificationTest.php
index 5aa24fc762..7517be715e 100644
--- a/tests/lib/Notification/NotificationTest.php
+++ b/tests/lib/Notification/NotificationTest.php
@@ -1,4 +1,5 @@
 <?php
+declare (strict_types = 1);
 /**
  * @author Joas Schilling <nickvergessen@owncloud.com>
  *
@@ -43,6 +44,7 @@ class NotificationTest extends TestCase {
 	protected function dataValidString($maxLength) {
 		$dataSets = [
 			['test1'],
+			['1564'],
 			[str_repeat('a', 1)],
 		];
 		if ($maxLength !== false) {
@@ -53,20 +55,24 @@ class NotificationTest extends TestCase {
 
 	protected function dataInvalidString($maxLength) {
 		$dataSets = [
-			[true],
-			[false],
-			[0],
-			[1],
-			[''],
-			[[]],
+			['']
 		];
 		if ($maxLength !== false) {
 			$dataSets[] = [str_repeat('a', $maxLength + 1)];
-			$dataSets[] = [[str_repeat('a', $maxLength + 1)]];
 		}
 		return $dataSets;
 	}
 
+	protected function dataInvalidStringType() {
+		return [
+			[true],
+			[false],
+			[16412],
+			[[]],
+			[null],
+		];
+	}
+
 	protected function dataInvalidInt() {
 		return [
 			[true],
@@ -98,6 +104,10 @@ class NotificationTest extends TestCase {
 		return $this->dataInvalidString(32);
 	}
 
+	public function dataSetAppInvalidType() {
+		return $this->dataInvalidStringType();
+	}
+
 	/**
 	 * @dataProvider dataSetAppInvalid
 	 * @param mixed $app
@@ -108,6 +118,17 @@ class NotificationTest extends TestCase {
 		$this->notification->setApp($app);
 	}
 
+	/**
+	 * @dataProvider dataSetAppInvalidType
+	 * @param mixed $app
+	 *
+	 * @expectedException \TypeError
+	 */
+	public function testSetAppInvalidType($app) {
+		$this->notification->setApp($app);
+	}
+
+
 	public function dataSetUser() {
 		return $this->dataValidString(64);
 	}
@@ -126,6 +147,10 @@ class NotificationTest extends TestCase {
 		return $this->dataInvalidString(64);
 	}
 
+	public function dataSetUserInvalidType() {
+		return $this->dataInvalidStringType();
+	}
+
 	/**
 	 * @dataProvider dataSetUserInvalid
 	 * @param mixed $user
@@ -136,6 +161,16 @@ class NotificationTest extends TestCase {
 		$this->notification->setUser($user);
 	}
 
+	/**
+	 * @dataProvider dataSetUserInvalidType
+	 * @param mixed $user
+	 *
+	 * @expectedException \TypeError
+	 */
+	public function testSetUserInvalidType($user) {
+		$this->notification->setUser($user);
+	}
+
 	public function dataSetDateTime() {
 		$past = new \DateTime();
 		$past->sub(new \DateInterval('P1Y'));