diff --git a/lib/6to5/register.js b/lib/6to5/register.js
index c5d51c833a..76cf325df7 100644
--- a/lib/6to5/register.js
+++ b/lib/6to5/register.js
@@ -127,7 +127,7 @@ var hookExtensions = function (_exts) {
   });
 };
 
-hookExtensions([".es6", ".es", ".js"]);
+hookExtensions(util.canCompile.EXTENSIONS);
 
 module.exports = function (opts) {
   // normalise options
diff --git a/lib/6to5/util.js b/lib/6to5/util.js
index 0affe7dab0..11301f16b4 100644
--- a/lib/6to5/util.js
+++ b/lib/6to5/util.js
@@ -12,11 +12,13 @@ var _          = require("lodash");
 exports.inherits = util.inherits;
 
 exports.canCompile = function (filename, altExts) {
-  var exts = altExts || [".js", ".jsx", ".es6", ".es"];
+  var exts = altExts || exports.canCompile.EXTENSIONS;
   var ext = path.extname(filename);
   return _.contains(exts, ext);
 };
 
+exports.canCompile.EXTENSIONS = [".js", ".jsx", ".es6", ".es"];
+
 exports.isInteger = function (i) {
   return _.isNumber(i) && i % 1 === 0;
 };