diff --git a/lib/6to5/util.js b/lib/6to5/util.js
index 9a1f25ba2e..3ab08d6900 100644
--- a/lib/6to5/util.js
+++ b/lib/6to5/util.js
@@ -91,7 +91,14 @@ exports.sourceMapToComment = function (map) {
 };
 
 exports.pushMutatorMap = function (mutatorMap, key, kind, method) {
-  var map = mutatorMap[key] = mutatorMap[key] || {};
+  var map;
+  if (_.has(mutatorMap, key)) {
+    map = mutatorMap[key];
+  } else {
+    map = {};
+  }
+  mutatorMap[key] = map;
+
   if (map[kind]) {
     throw new Error("a " + kind + " already exists for this property");
   } else {