diff --git a/bin/6to5/dir.js b/bin/6to5/dir.js
index ba98209960..af383a765d 100644
--- a/bin/6to5/dir.js
+++ b/bin/6to5/dir.js
@@ -17,8 +17,7 @@ module.exports = function (commander, filenames, opts) {
     var dest = path.join(commander.outDir, relative);
 
     var data = util.compile(src, {
-      // sourceMapName is the destination relative to the source
-      sourceMapName: path.relative(dest + '/..', src)
+      sourceFileName: path.relative(dest + '/..', src)
     });
 
     if (commander.sourceMaps) {
diff --git a/bin/6to5/util.js b/bin/6to5/util.js
index 14c63f009d..41d7cf326a 100644
--- a/bin/6to5/util.js
+++ b/bin/6to5/util.js
@@ -21,7 +21,7 @@ exports.addSourceMappingUrl = function (code, loc) {
 };
 
 exports.transform = function (filename, code, opts) {
-  opts = _.extend(opts || {}, index.opts);
+  opts = _.defaults(opts || {}, index.opts);
   opts.filename = filename;
 
   var result;