diff --git a/src/renderers/shared/hooks/ReactChildrenMutationWarningHook.js b/src/renderers/shared/hooks/ReactChildrenMutationWarningHook.js
index 0d4bd8767c..aa23b66bc1 100644
--- a/src/renderers/shared/hooks/ReactChildrenMutationWarningHook.js
+++ b/src/renderers/shared/hooks/ReactChildrenMutationWarningHook.js
@@ -7,6 +7,7 @@
  * of patent rights can be found in the PATENTS file in the same directory.
  *
  * @providesModule ReactChildrenMutationWarningHook
+ * @flow
  */
 
 'use strict';
@@ -15,6 +16,8 @@ var ReactComponentTreeHook = require('ReactComponentTreeHook');
 
 var warning = require('warning');
 
+import type { DebugID } from 'ReactInstanceType';
+
 function handleElement(debugID, element) {
   if (element == null) {
     return;
@@ -47,10 +50,10 @@ function handleElement(debugID, element) {
 }
 
 var ReactChildrenMutationWarningHook = {
-  onMountComponent(debugID) {
+  onMountComponent(debugID: DebugID): void {
     handleElement(debugID, ReactComponentTreeHook.getElement(debugID));
   },
-  onUpdateComponent(debugID) {
+  onUpdateComponent(debugID: DebugID): void {
     handleElement(debugID, ReactComponentTreeHook.getElement(debugID));
   },
 };