Can't get express to work with Redis

I'm using Express 4.x. I declare Redis here:

var app = require('express')()
, express = require('express')
, connect = require('connect')
, http = require('http').Server(app)
, path = require('path')
, request = require("request")
, io = require('socket.io')(http);


// Cookie and Session Reqs:
var bodyParser = require('body-parser');
var cookieParser = require('cookie-parser');
var session = require('express-session');
var RedisStore = require('connect-redis')(session);
var sessionStore = new RedisStore();

And I set up my session:

app.use(session({
    secret: "xxxxx",
    store: sessionStore,    
    key: 'sid',
    cookie: {secure: false, maxAge: 600000},
}));

And I use my session variable in one of my routes:

app.get('/public/', function(req, res) {
    req.session.sessionAccessCode = "not set yet";
    res.redirect("index.html");
});

When I remove the store: sessionStore line, there are no problems and I can test that the session variable works. But if I have it, I'm given an error:

TypeError: Cannot set property 'sessionAccessCode' of undefined

Any ideas why Redis isn't working?