When set maxAge to 'session', error hanppens with external redis store
Set maxAge to 'session'
const sesseionConfig = {
  key: sessionName,
  overwrite: false,
  maxAge: 'session',
  store: redisStore
}

The ttl param will be 'session' and then client.setex will be called with 'session' as the second param which should be a number. So redis client throw an error: 'ERR value is not an integer or out of range'

RedisStore.prototype.set = function *(sid, sess, ttl) {
  if (typeof ttl === 'number') {
    ttl = Math.ceil(ttl / 1000);
  }
  sess = JSON.stringify(sess);
  if (ttl) {
    debug('SETEX %s %s %s', sid, ttl, sess);
    yield this.client.setex(sid, ttl, sess);
  } else {
    debug('SET %s %s', sid, sess);
    yield this.client.set(sid, sess);
  }
  debug('SET %s complete', sid);
};