diff --git a/lib/6to5/file.js b/lib/6to5/file.js
index 46d42045bf..35d9fb22ba 100644
--- a/lib/6to5/file.js
+++ b/lib/6to5/file.js
@@ -39,8 +39,12 @@ File.normaliseOptions = function (opts) {
   opts.filename = opts.filename.replace(/\\/g, "/");
 
   _.defaults(opts, {
-    sourceFileName: opts.filename,
-    sourceMapName:  opts.filename
+    filenameRelative: opts.filename
+  });
+
+  _.defaults(opts, {
+    sourceFileName: opts.filenameRelative,
+    sourceMapName:  opts.filenameRelative
   });
 
   if (opts.runtime === true) {