Prefix seems to be broken

[batrudinych]

ubuntu 16.04 Node.js 10.15.1 koa 2.7.0 koa-joi-router 5.2.0

'use strict'

const Koa = require('koa')
const app = new Koa()

const router = require('koa-joi-router')

const orders = router()
orders.get('/', ctx => {
  ctx.body = 'Orders response'
})

orders.prefix('orders')

app.use(orders.middleware())

app.listen(3000)

Actual behavior:

localhost:3000/orders returns 404 Not Found

Expected behavior:

localhost:3000/orders returns 200 'Orders response`

Connected issue created in koa-router