Bug with koa-router nested routes with prefix

index.js

'use strict';

const koa    = require('koa');
const Router = require('koa-router');

let app = koa();

let router = new Router({
  prefix: '/prefix'
});

let fooRouter = new Router();

fooRouter.get('/', function *() {
  this.body = 'nested';
});

router.get('/', function *() {
  this.body = 'root';
});

router.all('/nested', fooRouter.routes());

app.use(router.routes());

app.listen(8888);