Express 500 Error: `secret` option required for sessions

app.js下

app.configure(function () {
    app.set('port', process.env.PORT || 3000);
    app.set('views', __dirname + '/views');
    app.set('view engine', 'jade');
    app.use(express.favicon());
    app.use(express.logger('dev'));
    app.use(express.bodyParser());
    app.use(express.methodOverride());
    app.use(express.cookieParser());
    app.use(express.session({
        secret: settings.cookiesSecret,
        store: new MongoStore({
            db: settings.db
        })
    }))
    app.use(app.router);
    app.use(express.static(path.join(__dirname, 'public')));
});

setting.js下：

module.exports = {
    cookieSecret: 'microblogbyvoid',
     db: 'microblog',
    host: 'localhost'
};

求帮助啊。。。。哪里有错啊。。。。。《node.js开发指南 》注册一章的例子呀