Golang negroni和http.NewServeMux（）问题

I am running a server with below code:

// Assuming there is no import error
  mux := http.NewServeMux()
  mux.HandleFunc("/", func(w http.ResponseWriter, req *http.Request) {
  http.Error(w, "File not found", http.StatusNotFound)
  })

  n := negroni.Classic()
  n.Use(negroni.HandlerFunc(bodmas.sum(4,5)))
  n.UseHandler(mux)
  n.Run(":4000" )

It works perfectly fine.

bodmas.sumhttp handler

  mux := http.NewServeMux()
  mux.HandleFunc("/", func(w http.ResponseWriter, req *http.Request) {
    http.Error(w, "File not found", http.StatusNotFound)
  })

  n := negroni.Classic()
  n.Use(negroni.HandlerFunc(wrapper.RateLimit(bodmas.sum(4,5),10)))
  n.UseHandler(mux)
  n.Run(":" + cfg.Server.Port)
}

wrapper.RateLimit

func RateLimit(h resized.HandlerFunc, rate int) (resized.HandlerFunc) {
    :
    :
  // logic here

    rl, _ := ratelimit.NewRateLimiter(rate)

    return func(w http.ResponseWriter, r *http.Request, next http.HandlerFunc){
        if rl.Limit(){
            http.Error(w, "Gateway Timeout", http.StatusGatewayTimeout )
        } else {
             next(w, r)
         }
    }
}

There is no error. Any suggestions about this behavior ? How to make it work ?