diff --git a/lib/6to5/transformation/modules/_default.js b/lib/6to5/transformation/modules/_default.js
index 98b5043d22..bf5d35d17f 100644
--- a/lib/6to5/transformation/modules/_default.js
+++ b/lib/6to5/transformation/modules/_default.js
@@ -161,6 +161,9 @@ DefaultFormatter.prototype.getModuleName = function () {
 
   moduleName += filenameRelative;
 
+  // normalise path separators
+  moduleName = moduleName.replace(/\\/g, "/");
+
   return moduleName;
 };