diff --git a/web-locks/web-locks.idl b/web-locks/web-locks.idl
index 351c1c5d822032..14bc3a22cc395f 100644
--- a/web-locks/web-locks.idl
+++ b/web-locks/web-locks.idl
@@ -16,7 +16,7 @@ interface LockManager {
   Promise<LockManagerSnapshot> query();
 };
 
-callback LockGrantedCallback = Promise<any> (Lock lock);
+callback LockGrantedCallback = Promise<any> (Lock? lock);
 
 enum LockMode { "shared", "exclusive" };