Express get lost in co() context --WRONG

minimal.js

var express = require('express');

var app = express();
var co = require('co');

function * sleep(){
  var p = new Promise(function(w,f){
    setTimeout(w,1000,'winning');
  });
  return yield p;
}

app.get('/',function(req,res) {
  co(function*(){
    console.log('inside co');
    var val = yield sleep();
    console.log('sleep done', val);
    res.send(val);
    console.log('it does reach here.');
  })
});

module.exports = app;

var port = process.env.PORT || 4000;

app.listen(port);
console.log('Express view app listening on port ' + port);