diff --git a/lib/6to5/file.js b/lib/6to5/file.js
index 5c953ba7f2..4fb9e05476 100644
--- a/lib/6to5/file.js
+++ b/lib/6to5/file.js
@@ -107,10 +107,6 @@ File.normaliseOptions = function (opts) {
   opts.optional  = util.arrayify(opts.optional);
   opts.loose     = util.arrayify(opts.loose);
 
-  opts.optional = opts.optional.map(function (key) {
-    return "optional." + key;
-  });
-
   if (_.contains(opts.loose, "all")) {
     opts.loose = Object.keys(transform.transformers);
   }