diff --git a/lib/_stream_duplex.js b/lib/_stream_duplex.js
index b96a0439f785fa..27686b1490f4e5 100644
--- a/lib/_stream_duplex.js
+++ b/lib/_stream_duplex.js
@@ -39,7 +39,7 @@ Object.setPrototypeOf(Duplex, Readable);
 {
   // Allow the keys array to be GC'ed.
   const keys = Object.keys(Writable.prototype);
-  for (var v = 0; v < keys.length; v++) {
+  for (let v = 0; v < keys.length; v++) {
     const method = keys[v];
     if (!Duplex.prototype[method])
       Duplex.prototype[method] = Writable.prototype[method];