diff --git a/lib/_stream_duplex.js b/lib/_stream_duplex.js
index 6752519225..53f3171e0b 100644
--- a/lib/_stream_duplex.js
+++ b/lib/_stream_duplex.js
@@ -57,6 +57,7 @@ require('inherits')(Duplex, Readable);
 
 function Duplex(options) {
   if (!(this instanceof Duplex)) return new Duplex(options);
+  this.isDuplex = true
   Readable.call(this, options);
   Writable.call(this, options);
   this.allowHalfOpen = true;
diff --git a/lib/_stream_readable.js b/lib/_stream_readable.js
index 192d451488..2b54f27a6a 100644
--- a/lib/_stream_readable.js
+++ b/lib/_stream_readable.js
@@ -21,10 +21,6 @@
 'use strict';
 
 module.exports = Readable;
-/*<replacement>*/
-
-var Duplex;
-/*</replacement>*/
 
 Readable.ReadableState = ReadableState;
 /*<replacement>*/
@@ -103,16 +99,14 @@ function prependListener(emitter, event, fn) {
   if (!emitter._events || !emitter._events[event]) emitter.on(event, fn);else if (Array.isArray(emitter._events[event])) emitter._events[event].unshift(fn);else emitter._events[event] = [fn, emitter._events[event]];
 }
 
-function ReadableState(options, stream, isDuplex) {
-  Duplex = Duplex || require('./_stream_duplex');
+function ReadableState(options,isDuplex) {
   options = options || {}; // Duplex streams are both readable and writable, but share
   // the same options object.
   // However, some cases require setting options to different
   // values for the readable and the writable sides of the duplex stream.
   // These options can be provided separately as readableXXX and writableXXX.
 
-  if (typeof isDuplex !== 'boolean') isDuplex = stream instanceof Duplex; // object stream flag. Used to make read(n) ignore n and to
-  // make all the buffer merging and length checks go away
+    // make all the buffer merging and length checks go away
 
   this.objectMode = !!options.objectMode;
   if (isDuplex) this.objectMode = this.objectMode || !!options.readableObjectMode; // the point at which it stops calling _read() to fill the buffer
@@ -167,12 +161,11 @@ function ReadableState(options, stream, isDuplex) {
 }
 
 function Readable(options) {
-  Duplex = Duplex || require('./_stream_duplex');
+  
   if (!(this instanceof Readable)) return new Readable(options); // Checking for a Stream.Duplex instance is faster here instead of inside
   // the ReadableState constructor, at least with V8 6.5
-
-  var isDuplex = this instanceof Duplex;
-  this._readableState = new ReadableState(options, this, isDuplex); // legacy
+  
+  this._readableState = new ReadableState(options, this.isDuplex); // legacy
 
   this.readable = true;