node.js socket.io 与 express 集成出错

浏览器端

var socket = io.connect('http://localhost:3000');

服务器端

var express = require('express');
var routes = require('./routes/index');
var user = require('./routes/user');
var chat = require('./routes/chat');
var http = require('http');
var path = require('path');
var MongoStore = require('connect-mongo')(express);
var settings = require('./settings');
var flash = require('connect-flash');
var multer  = require('multer');

var app = express();

app.set('port', process.env.PORT || 3000);
app.set('views', path.join(__dirname, 'views'));
app.set('view engine', 'ejs');
app.use(flash());
app.use(express.favicon());
app.use(express.logger('dev'));
app.use(express.json());
app.use(express.urlencoded());
app.use(express.methodOverride());
app.use(express.cookieParser());
app.use(express.session({
  secret:settings.cookiesSecret,
  key:settings.db,
  cookie:{maxAge:1000*60*60*24*30},
  store:new MongoStore({
    db:settings.db
  })
}));

app.use(multer({
  dest: './public/images/user',
  rename: function (fieldname, filename) {
    return filename;
  }
}));

app.use(app.router);
app.use(express.static(path.join(__dirname, 'public')));
app.use(express.static(path.join(__dirname,'bower_components')));


if ('development' == app.get('env')) {
  app.use(express.errorHandler());
}

routes(app);

var server =  http.createServer(app);
var io = require('socket.io')(server);
io.on('connection', function(){ console.log('connection') });
server.listen(app.get('port'), function(){
  console.log('Express server listening on port ' + app.get('port'));
});

出错这个错误：

Request URL:http://localhost:3000/socket.io/1/?t=1420622609420
Status Code:400 Bad Request

服务器返回的错误信息是：{"code":0,"message":"Transport unknown"}

这是什么情况？求大神解答~~~