diff --git a/lib/6to5/types/index.js b/lib/6to5/types/index.js
index 284fb7eb12..efe141328f 100644
--- a/lib/6to5/types/index.js
+++ b/lib/6to5/types/index.js
@@ -3,6 +3,10 @@ var _       = require("lodash");
 
 var t = exports;
 
+t.NATIVE_TYPES_NAMES = ["Array", "Object", "Number", "Boolean", "Date", "Array", "String"];
+
+//
+
 var addAssert = function (type, is) {
   t["assert" + type] = function (node, opts) {
     opts = opts || {};
@@ -214,7 +218,7 @@ t.isReferenced = function (node, parent) {
  */
 
 t.isValidIdentifier = function (name) {
-  return _.isString(name) && esutils.keyword.isIdentifierName(name) && !esutils.keyword.isKeywordES6(name, true);
+  return _.isString(name) && esutils.keyword.isIdentifierName(name) && !esutils.keyword.isReservedWordES6(name, true);
 };
 
 /*
@@ -227,6 +231,8 @@ t.isValidIdentifier = function (name) {
 t.toIdentifier = function (name) {
   if (t.isIdentifier(name)) return name.name;
 
+  name = name + "";
+
   // replace all non-valid identifiers with dashes
   name = name.replace(/[^a-zA-Z0-9$_]/g, "-");
 
@@ -241,6 +247,10 @@ t.toIdentifier = function (name) {
   // remove underscores from start of name
   name = name.replace(/^\_/, "");
 
+  if (!t.isValidIdentifier(name)) {
+    name = "_" + name;
+  }
+
   return name || '_';
 };