diff --git a/index.js b/index.js
index 6ce47c6..9e7aa28 100644
--- a/index.js
+++ b/index.js
@@ -59,7 +59,7 @@ module.exports = (function(doc, win) {
     }
 
     function mergeObjects(targetObj, sourceObject) {
-        for (key in sourceObject) {
+        for (var key in sourceObject) {
             if (sourceObject.hasOwnProperty(key)) {
                 targetObj[key] = sourceObject[key];
             }