diff --git a/lib/6to5/register.js b/lib/6to5/register.js
index d34f447533..9dbd1bf589 100644
--- a/lib/6to5/register.js
+++ b/lib/6to5/register.js
@@ -1,6 +1,7 @@
 require("./polyfill");
 
 var sourceMapSupport = require("source-map-support");
+var util             = require("./util");
 var to5              = require("./index");
 var _                = require("lodash");
 
@@ -100,11 +101,11 @@ module.exports = function (opts) {
   if (_.isRegExp(opts)) opts = { ignore: opts };
   if (opts.ignoreRegex != null) opts.ignore = opts.ignoreRegex;
 
-  if (opts.only != null) onlyRegex = opts.only;
-  if (opts.ignore != null) ignoreRegex = opts.ignore;
+  if (opts.only != null) onlyRegex = util.regexify(opts.only);
+  if (opts.ignore != null) ignoreRegex = util.regexify(opts.ignore);
 
-  if (opts.extensions) hookExtensions(opts.extensions);
+  if (opts.extensions) hookExtensions(util.arrayify(opts.extensions));
 
-  if (opts.blacklist) blacklist = opts.blacklist;
-  if (opts.whitelist) whitelist = opts.whitelist;
+  if (opts.blacklist) blacklist = util.arrayify(opts.blacklist);
+  if (opts.whitelist) whitelist = util.arrayify(opts.whitelist);
 };