diff --git a/apps/dav/tests/unit/Connector/Sabre/AddExtraHeadersPluginTest.php b/apps/dav/tests/unit/Connector/Sabre/AddExtraHeadersPluginTest.php
index 2906afea7d8..f913010773b 100644
--- a/apps/dav/tests/unit/Connector/Sabre/AddExtraHeadersPluginTest.php
+++ b/apps/dav/tests/unit/Connector/Sabre/AddExtraHeadersPluginTest.php
@@ -81,9 +81,13 @@ class AddExtraHeadersPluginTest extends TestCase {
 		$this->tree->expects($this->once())->method('getNodeForPath')
 			->willReturn($node);
 
-		$user = $this->createMock(IUser::class);
-		$node->expects($this->once())->method('getOwner')->willReturn($user);
-		$user->expects($this->once())->method('getUID')->willReturn($ownerId);
+		if ($ownerId !== null) {
+			$user = $this->createMock(IUser::class);
+			$node->expects($this->once())->method('getOwner')->willReturn($user);
+			$user->expects($this->once())->method('getUID')->willReturn($ownerId);
+		} else {
+			$node->expects($this->once())->method('getOwner')->willReturn(null);
+		}
 		$node->expects($this->once())->method('getDavPermissions')->willReturn($permissions);
 
 		$matcher = $this->exactly($expectedInvocations);
diff --git a/apps/dav/tests/unit/Connector/Sabre/PrincipalTest.php b/apps/dav/tests/unit/Connector/Sabre/PrincipalTest.php
index 8699903e1ec..8cd19b1143c 100644
--- a/apps/dav/tests/unit/Connector/Sabre/PrincipalTest.php
+++ b/apps/dav/tests/unit/Connector/Sabre/PrincipalTest.php
@@ -101,6 +101,10 @@ class PrincipalTest extends TestCase {
 			->expects($this->once())
 			->method('getSystemEMailAddress')
 			->willReturn('bar@nextcloud.com');
+		$barUser
+			->expects($this->once())
+			->method('getDisplayName')
+			->willReturn('bar');
 		$this->userManager
 			->expects($this->once())
 			->method('search')
@@ -189,6 +193,10 @@ class PrincipalTest extends TestCase {
 			->expects($this->once())
 			->method('getUID')
 			->willReturn('foo');
+		$fooUser
+			->expects($this->once())
+			->method('getDisplayName')
+			->willReturn('foo');
 		$this->userManager
 			->expects($this->once())
 			->method('get')
@@ -221,6 +229,10 @@ class PrincipalTest extends TestCase {
 			->expects($this->once())
 			->method('getUID')
 			->willReturn('foo');
+		$fooUser
+			->expects($this->once())
+			->method('getDisplayName')
+			->willReturn('foo');
 		$this->userManager
 			->expects($this->once())
 			->method('get')
diff --git a/apps/settings/tests/Controller/UsersControllerTest.php b/apps/settings/tests/Controller/UsersControllerTest.php
index 39a4dbc42c0..1a47fa5b74f 100644
--- a/apps/settings/tests/Controller/UsersControllerTest.php
+++ b/apps/settings/tests/Controller/UsersControllerTest.php
@@ -675,7 +675,7 @@ class UsersControllerTest extends \Test\TestCase {
 	}
 
 	#[\PHPUnit\Framework\Attributes\DataProvider(methodName: 'dataTestSaveUserSettings')]
-	public function testSaveUserSettings(array $data, ?string $oldEmailAddress, ?string $oldDisplayName): void {
+	public function testSaveUserSettings(array $data, ?string $oldEmailAddress, string $oldDisplayName): void {
 		$controller = $this->getController();
 		$user = $this->createMock(IUser::class);
 
@@ -690,7 +690,7 @@ class UsersControllerTest extends \Test\TestCase {
 				->with($data[IAccountManager::PROPERTY_EMAIL]['value']);
 		}
 
-		if ($data[IAccountManager::PROPERTY_DISPLAYNAME]['value'] === $oldDisplayName ?? '') {
+		if ($data[IAccountManager::PROPERTY_DISPLAYNAME]['value'] === $oldDisplayName) {
 			$user->expects($this->never())->method('setDisplayName');
 		} else {
 			$user->expects($this->once())->method('setDisplayName')
@@ -775,7 +775,7 @@ class UsersControllerTest extends \Test\TestCase {
 					IAccountManager::PROPERTY_DISPLAYNAME => ['value' => 'john doe'],
 				],
 				'john@example.com',
-				null
+				''
 			],
 			[
 				[
@@ -783,7 +783,7 @@ class UsersControllerTest extends \Test\TestCase {
 					IAccountManager::PROPERTY_DISPLAYNAME => ['value' => 'john doe'],
 				],
 				'JOHN@example.com',
-				null
+				''
 			],
 		];
 	}
diff --git a/tests/lib/Contacts/ContactsMenu/Providers/LocalTimeProviderTest.php b/tests/lib/Contacts/ContactsMenu/Providers/LocalTimeProviderTest.php
index a36416ca9ea..e595af8e035 100644
--- a/tests/lib/Contacts/ContactsMenu/Providers/LocalTimeProviderTest.php
+++ b/tests/lib/Contacts/ContactsMenu/Providers/LocalTimeProviderTest.php
@@ -186,10 +186,6 @@ class LocalTimeProviderTest extends TestCase {
 			->with('UID')
 			->willReturn('user1');
 
-		$user = $this->createMock(IUser::class);
-		$user->method('getUID')
-			->willReturn(null);
-
 		$entry->expects($this->never())
 			->method('addAction');
 
diff --git a/tests/lib/TaskProcessing/TaskProcessingTest.php b/tests/lib/TaskProcessing/TaskProcessingTest.php
index 3bfc586240c..bb47bd735b9 100644
--- a/tests/lib/TaskProcessing/TaskProcessingTest.php
+++ b/tests/lib/TaskProcessing/TaskProcessingTest.php
@@ -912,7 +912,7 @@ class TaskProcessingTest extends \Test\TestCase {
 			new ServiceRegistration('test', AsyncProvider::class)
 		]);
 		$user = $this->createMock(IUser::class);
-		$user->expects($this->any())->method('getUID')->willReturn(null);
+		$user->expects($this->any())->method('getUID')->willReturn('uid');
 		$mount = $this->createMock(ICachedMountInfo::class);
 		$mount->expects($this->any())->method('getUser')->willReturn($user);
 		$this->userMountCache->expects($this->any())->method('getMountsForFileId')->willReturn([$mount]);