diff --git a/index.bs b/index.bs
index 88922ba..f7f0b33 100644
--- a/index.bs
+++ b/index.bs
@@ -326,7 +326,7 @@ interface LockManager {
   Promise<LockManagerSnapshot> query();
 };
 
-callback LockGrantedCallback = Promise<any> (Lock lock);
+callback LockGrantedCallback = Promise<any> (Lock? lock);
 
 enum LockMode { "shared", "exclusive" };