session store is unavaliable

What is this mean, i just copy your code and run it.

var session = require('koa-generic-session');
var redisStore = require('koa-redis');
var koa = require('koa');

var app = koa();
app.keys = ['keys', 'keykeys'];
app.use(session({
  store: redisStore()
}));
...

Error: session store is unavaliable
      at null._onTimeout (/Users/jiangxingshang/Documents/workspace/WebStorm/Shop/node_modules/koa-generic-session/lib/session.js:105:16)
      at Timer.listOnTimeout (timers.js:110:15)

[dead-horse]

did you start your redis server?

[smz8023]

你好，我在使用过程中，报错： server error { TypeError: Cannot set property name of which has only a getter at defaultErrorHanlder