diff --git a/lib/6to5/transformation/file.js b/lib/6to5/transformation/file.js
index b41b853319..2eccb48b3a 100644
--- a/lib/6to5/transformation/file.js
+++ b/lib/6to5/transformation/file.js
@@ -123,7 +123,10 @@ File.prototype.normalizeOptions = function (opts) {
   });
 
   // normalize windows path separators to unix
-  opts.filename = opts.filename.replace(/\\/g, "/");
+  opts.filename = util.normalisePathSeparator(opts.filename);
+  if (opts.sourceRoot) {
+    opts.sourceRoot = util.normalisePathSeparator(opts.sourceRoot);
+  }
 
   opts.basename = path.basename(opts.filename, path.extname(opts.filename));
 
diff --git a/lib/6to5/util.js b/lib/6to5/util.js
index 501315617f..5afe1e97e1 100644
--- a/lib/6to5/util.js
+++ b/lib/6to5/util.js
@@ -30,6 +30,10 @@ exports.canCompile = function (filename, altExts) {
 
 exports.canCompile.EXTENSIONS = [".js", ".jsx", ".es6", ".es"];
 
+exports.normalisePathSeparator = function (filename) {
+  return filename.replace(/\\/g, "/");
+};
+
 exports.isInteger = function (i) {
   return isNumber(i) && i % 1 === 0;
 };