diff --git a/apps/dav/lib/CardDAV/AddressBookImpl.php b/apps/dav/lib/CardDAV/AddressBookImpl.php
index ae77498539ba8..00065d49b1755 100644
--- a/apps/dav/lib/CardDAV/AddressBookImpl.php
+++ b/apps/dav/lib/CardDAV/AddressBookImpl.php
@@ -41,7 +41,7 @@ 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 74699cf3925da..77b5db309dffa 100644
--- a/apps/dav/tests/unit/CardDAV/AddressBookImplTest.php
+++ b/apps/dav/tests/unit/CardDAV/AddressBookImplTest.php
@@ -53,7 +53,8 @@ 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());
 	}