为什么只能渲染根目录

能请求到
app.use(convert(mount('/', serve(`${process.cwd()}/src/client`))))
请求不到
app.use(convert(mount('/aa', serve(`${process.cwd()}/src/client`))))
错误提示
TypeError: next is not a function
      at Object.serve (/Users/yangyss/Desktop/Cool/react-koa/node_modules/koa-static/index.js:47:14)
      at Object.<anonymous> (/Users/yangyss/Desktop/Cool/react-koa/node_modules/koa-mount/index.js:58:23)
      at Generator.next (<anonymous>)
      at onFulfilled (/Users/yangyss/Desktop/Cool/react-koa/node_modules/co/index.js:65:19)
      at /Users/yangyss/Desktop/Cool/react-koa/node_modules/co/index.js:54:5
      at Object.co (/Users/yangyss/Desktop/Cool/react-koa/node_modules/co/index.js:50:10)
      at converted (/Users/yangyss/Desktop/Cool/react-koa/node_modules/koa-convert/index.js:17:15)
      at dispatch (/Users/yangyss/Desktop/Cool/react-koa/node_modules/koa-compose/index.js:44:32)
      at next (/Users/yangyss/Desktop/Cool/react-koa/node_modules/koa-compose/index.js:45:18)
      at middleware (/Users/yangyss/Desktop/Cool/react-koa/node_modules/koa-webpack-middleware/node_modules/webpack-hot-middleware/middleware.js:26:48)
      at _callee$ (/Users/yangyss/Desktop/Cool/react-koa/node_modules/koa-webpack-middleware/lib/hotMiddleware.js:30:22)
      at tryCatch (/Users/yangyss/Desktop/Cool/react-koa/node_modules/regenerator-runtime/runtime.js:64:40)
      at Generator.invoke [as _invoke] (/Users/yangyss/Desktop/Cool/react-koa/node_modules/regenerator-runtime/runtime.js:299:22)
      at Generator.prototype.(anonymous function) [as next] (/Users/yangyss/Desktop/Cool/react-koa/node_modules/regenerator-runtime/runtime.js:116:21)
      at step (/Users/yangyss/Desktop/Cool/react-koa/node_modules/koa-webpack-middleware/lib/hotMiddleware.js:15:191)
      at /Users/yangyss/Desktop/Cool/react-koa/node_modules/koa-webpack-middleware/lib/hotMiddleware.js:15:437

详细代码： https://github.com/dlyt/react-koa/blob/master/bin/server.js