error when set session
 Error: .keys required for signed cookies

var session = require('koa-session');
router.get('/getScore', koaBody, session(app),function *(){
        var n = this.session.views || 0;
        this.session.views = ++n;
        this.body = n + ' views';
})
app.use(convert(session( app)));

Help me please! thk
