diff --git a/packages/test-utils/src/set-watchers-to-sync.js b/packages/test-utils/src/set-watchers-to-sync.js
index 56bbf3ff9..b6b2a0ee0 100644
--- a/packages/test-utils/src/set-watchers-to-sync.js
+++ b/packages/test-utils/src/set-watchers-to-sync.js
@@ -27,6 +27,7 @@ export function setWatchersToSync (vm) {
 
   vm.$children.forEach(setWatchersToSync)
 
+  // preventing double registration
   if (!vm.$_vueTestUtils_updateInSetWatcherSync) {
     vm.$_vueTestUtils_updateInSetWatcherSync = vm._update
     vm._update = function (vnode, hydrating) {