cannot call method 'on' of undefined at object

Hi guys when i do when i try to run node app.js in my node.js command prompt i get an error saying cannot call method 'on' of undefined at object heres my code

var express = require('express'),
app = express(),
server = require('http').createServer(app),
io = require('socket.io').listen(server);

    server.listen(3000);

app.get('/', function(req, res) {
res.sendfile(__dirname + '/index.html');
});

io.socket.on('connection', function(socket) {
socket.on('send message', function(data){
    io.socket.emit('new message', data);
});
});

i dont know whats wrong