Express Js: Router.use() Requires Middleware function

I am developing an application in Express Js. When I try to run the application I get this error:

My app.js file is like this:

var express = require('express'); var path = require('path'); var favicon = require('serve-favicon'); var logger = require('morgan'); var cookieParser = require('cookie-parser'); var bodyParser = require('body-parser'); var routes = require('./routes/index'); var users = require('./routes/users'); var app = express(); app.set('views', path.join(__dirname, 'views')); app.set('view engine', 'jade'); app.use(logger('dev')); app.use(bodyParser.json()); app.use(bodyParser.urlencoded({ extended: false })); app.use(cookieParser()); app.use(express.static(path.join(__dirname, 'public'))); app.use('/', routes); app.use('/users', users); // catch 404 and forward to error handler app.use(function(req, res, next) { var err = new Error('Not Found'); err.status = 404; next(err); }); if (app.get('env') === 'development') { app.use(function(err, req, res, next) { res.status(err.status || 500); res.render('error', { message: err.message, error: err }); }); } app.use(function(err, req, res, next) { res.status(err.status || 500); res.render('error', { message: err.message, error: {} }); }); var server = app.listen(3000, function() { var host = server.address().address; var port = server.address().port; console.log('Example app listening at http://%s:%s', host, port); }); module.exports = router; 

My index.js is like this:

 var express = require('express'); var router = express.Router(); /* GET home page. */ router.get('/', function (req, res, next) { res.render('index', {title: 'Polls'}); }); router.get("/list", function (req, res, next) { Poll.find({}, 'question', function (error, polls) { res.json(polls); }); }); router.get("/poll", function (req, res, next) { var pollId = req.params.id; // Find the poll by its ID, use lean as we won't be changing it Poll.findById(pollId, '', {lean: true}, function (err, poll) { if (poll) { var userVoted = false, userChoice, totalVotes = 0; // Loop through poll choices to determine if user has voted // on this poll, and if so, what they selected for (c in poll.choices) { var choice = poll.choices[c]; for (v in choice.votes) { var vote = choice.votes[v]; totalVotes++; if (vote.ip === (req.header('x-forwarded-for') || req.ip)) { userVoted = true; userChoice = {_id: choice._id, text: choice.text}; } } } // Attach info about user's past voting on this poll poll.userVoted = userVoted; poll.userChoice = userChoice; poll.totalVotes = totalVotes; res.json(poll); } else { res.json({error: true}); } }); }); router.get("/create", function (req, res, next) { var reqBody = req.body, // Filter out choices with empty text choices = reqBody.choices.filter(function (v) { return v.text != ''; }), // Build up poll object to save pollObj = {question: reqBody.question, choices: choices}; // Create poll model from built up poll object var poll = new Poll(pollObj); // Save poll to DB poll.save(function (err, doc) { if (err || !doc) { throw 'Error'; } else { res.json(doc); } }); }); module.exports = router; 

And user.js is this:

var express = require('express'); var router = express.Router(); /* GET users listing. */ router.get('/', function(req, res, next) { res.send('respond with a resource'); }); module.exports = router; 

I tried to find my solution on SO, but couldn't. Feel free to tell me if i need to provide any other file. Any help?