diff --git a/lib/6to5/file.js b/lib/6to5/file.js
index 2f5127f350..291b6601ef 100644
--- a/lib/6to5/file.js
+++ b/lib/6to5/file.js
@@ -33,6 +33,9 @@ File.normaliseOptions = function (opts) {
     runtime:    false
   });
 
+  // normalise windows path separators to unix
+  opts.filename = opts.filename.replace(/\\/g, "/");
+
   _.defaults(opts, {
     sourceFileName: opts.filename,
     sourceMapName:  opts.filename