diff --git a/lib/internal/streams/duplex.js b/lib/internal/streams/duplex.js
index bc92e9021ab57c..79f91fa0866e25 100644
--- a/lib/internal/streams/duplex.js
+++ b/lib/internal/streams/duplex.js
@@ -42,8 +42,10 @@ ObjectSetPrototypeOf(Duplex.prototype, Readable.prototype);
 ObjectSetPrototypeOf(Duplex, Readable);
 
 {
+  const keys = ObjectKeys(Writable.prototype);
   // Allow the keys array to be GC'ed.
-  for (const method of ObjectKeys(Writable.prototype)) {
+  for (let i = 0; i < keys.length; i++) {
+    const method = keys[i];
     if (!Duplex.prototype[method])
       Duplex.prototype[method] = Writable.prototype[method];
   }