Unable to connect to redis using TLS

I am using koa-session with koa-redis to store some session data in a redis store. The redis store is hosted in Azure and has to be accessed using TLS.

When I run my code against a local redis server it works correctly, even with a redis config that requires a password. However, when I try to connect to the Azure redis instance I am unable to connect.

Here is the options I am passing to create the redis store. I believe these are correct based on the documentation:

const options = {
    key: 'pages:session',
    store: redisStore({
        port: config.auth.redisPort,
        host: config.auth.redisHost,
        auth_pass: config.auth.redisPassword,
        tls: {
            servername: config.auth.redisHost
        }
    })
};

app.use(session(options, app));

Any thoughts as to what the issue might be would be appreciated.