express connect.session

Warning: connect.session() MemoryStore is not
designed for a production environment, as it will leak

这是什么错，express的session配置如下

app.use(session({
    secret: 'secret key',
    cookie: {maxAge: 12 * 3600 * 1000},
    resave: true,
    saveUninitialized: true,
    rolling: true
}));