diff --git a/src/browser/ui/React.js b/src/browser/ui/React.js
index 64143e32e8..f12ed7dbf5 100644
--- a/src/browser/ui/React.js
+++ b/src/browser/ui/React.js
@@ -130,7 +130,7 @@ if (__DEV__) {
       Object.freeze
     ];
 
-    for (var i in expectedFeatures) {
+    for (var i = 0; i < expectedFeatures.length; i++) {
       if (!expectedFeatures[i]) {
         console.error(
           'One or more ES5 shim/shams expected by React are not available: ' +