Express Session: Object #<Object> has no method 'on'

I'm using ExpressJS and have been trying to get PassportJS working. In my express.js config, I have the following:

app.configure(function () {
        // dynamic helpers
        app.use(helpers(config.app.name));

        // cookieParser should be above session
        app.use(express.cookieParser());

        // bodyParser should be above methodOverride
        app.use(express.bodyParser());
        app.use(express.methodOverride());

        // express/mongo session storage
        app.use(express.session({
          secret: 'noobjs',
          store: new mongoStore({
            url: config.db,
            collection : 'sessions'
          })
        }));

        // connect flash for flash messages
        app.use(flash());

        // use passport session
        app.use(passport.initialize());
        app.use(passport.session());

...

Unfortunately, I get the following error when I run the app:-

store.on('disconnect', function(){ storeReady = false; });
^
TypeError: Object #<Object> has no method 'on'

I think it has something to do with initialization order, but am too inexperienced around express to know what the order should be.

Can anyone offer some direction please?