Почему koa + socket.io выдаёт ошибку — TypeError: Class constructor Application cannot be invoked without 'new'?

const
    http = require('http'),
    koa = require('koa'),
    app = koa(),
    serve = require('koa-static'),
    server = http.createServer(app.callback()),
    io = require('socket.io')(server);


app.use(serve('./public'));

io.on('connection', function (socket) {
  socket.on('click', function (data) {
    //process the data here
    console.log('client clicked! data:');
    console.log(data);

    // emit an event
    console.log('responding with news');
    socket.emit('news', { hello: 'world' });
  });
});

var port = process.env.PORT || 3000;
server.listen(port);
console.log ('Listening at port ' + port + ' ...');