diff --git a/README.md b/README.md
index 4542368..4500c68 100644
--- a/README.md
+++ b/README.md
@@ -60,7 +60,7 @@ async function markAllComplete(tx: WriteTransaction) {
     // Mutators are transactional, so this is safe. The entire function will
     // run atomically and `t.complete` cannot change while it is running.
     if (!t.complete) {
-      await updateTodo(todo.id, {complete: true});
+      await updateTodo(tx, {id: todo.id, complete: true});
     }
   }
 }