diff --git a/lib/private/Files/Config/UserMountCache.php b/lib/private/Files/Config/UserMountCache.php
index 4b676092dfe..d32e699ab89 100644
--- a/lib/private/Files/Config/UserMountCache.php
+++ b/lib/private/Files/Config/UserMountCache.php
@@ -353,6 +353,7 @@ class UserMountCache implements IUserMountCache {
 	 * @since 9.0.0
 	 */
 	public function getMountsForFileId($fileId, $user = null) {
+		$fileId = (int) $fileId;
 		try {
 			[$storageId, $internalPath] = $this->getCacheInfoFromFileId($fileId);
 		} catch (NotFoundException $e) {