koa-router 无法获取路径参数

Question

环境
Node 8.0.0
koa 2.2.0
koa-router 7.1.1

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

router.prefix('/logs/mobile')

router.get('/:platform', function(cxt, next) {

    console.log('params', this.params);

    cxt.body = { error: 'test'}

})

module.exports = router

代码如上所示,然后访问 http://127.0.0.1:9901/logs/mobile/ios 成功获取了body的返回值,
但是this.params为undefined