own through stream does not pipe data

I'm learning about node.js streams at the moment and therefore I'd like to write my own custom streaming module. Here is what I have at the moment:

index.js:

fs = require("fs")
var module = require("./module")
var ws = fs.createWriteStream("out.txt")

fs.createReadStream("in.txt")
  .pipe(module())
  .pipe(ws)

module.js:

var Stream = require('stream').Stream;

module.exports = function() {
  var stream = new Stream();

  this.writable = true;
  this.readable = true;

  stream.write = function(data) {
    console.log("write")
    stream.emit("data", data)
  }

  stream.end = function() {
    console.log("end")
    stream.emit("end")
  }

  stream.destroy = function() {
    console.log("destroy")
    stream.emit("close")
  }

  return stream;
}

in.txt:

text
text
text
text
text
text
text
text
text
text

The problem is that the out.txt is empty and the stream.write function is never called. Why is that so? If I comment out my middle pipe is all works fine!