diff --git a/script/lib/s3put.js b/script/lib/s3put.js
index 781b906c8043..8f92b6d95dec 100644
--- a/script/lib/s3put.js
+++ b/script/lib/s3put.js
@@ -13,7 +13,7 @@ if (prefix && !prefix.endsWith(path.sep)) prefix = path.resolve(prefix) + path.s
 function filenameToKey (file) {
   file = path.resolve(file);
   if (file.startsWith(prefix)) file = file.substr(prefix.length - 1);
-  return key_prefix + file.replace(path.sep, '/');
+  return key_prefix + (path.sep === '\\' ? file.replace(/\\/g, '/') : file);
 }
 
 let anErrorOccurred = false;