diff --git a/apps/dav/lib/CardDAV/AddressBookImpl.php b/apps/dav/lib/CardDAV/AddressBookImpl.php
index fd42447f0e539..186f81d172d25 100644
--- a/apps/dav/lib/CardDAV/AddressBookImpl.php
+++ b/apps/dav/lib/CardDAV/AddressBookImpl.php
@@ -66,7 +66,7 @@ public function __construct(
 	 * @since 5.0.0
 	 */
 	public function getKey() {
-		return $this->addressBookInfo['id'];
+		return (string) $this->addressBookInfo['id'];
 	}
 
 	/**
diff --git a/apps/dav/tests/unit/CardDAV/AddressBookImplTest.php b/apps/dav/tests/unit/CardDAV/AddressBookImplTest.php
index 038bc0274d9dc..f9563991af23a 100644
--- a/apps/dav/tests/unit/CardDAV/AddressBookImplTest.php
+++ b/apps/dav/tests/unit/CardDAV/AddressBookImplTest.php
@@ -67,7 +67,8 @@ protected function setUp(): void {
 	}
 
 	public function testGetKey(): void {
-		$this->assertSame($this->addressBookInfo['id'],
+		$this->assertIsString($this->addressBookImpl->getKey());
+		$this->assertSame((string) $this->addressBookInfo['id'],
 			$this->addressBookImpl->getKey());
 	}