golang net/http panic when using timeout handler

timeout.go

package main

import (
	"flag"
	"io/ioutil"
	"log"
	"net"
	"net/http"
	"time"
)

var (
	addr    = flag.String("addr", ":8080", "address to listen HTTP")
	timeout = flag.Duration("timeout", 200*time.Millisecond, "timeout duration")
)

func main() {
	flag.Parse()

	handler := http.TimeoutHandler(http.HandlerFunc(FileReadingHandler), *timeout, "http timeout")
	http.Handle("/", handler)

	listener, err := net.Listen("tcp", *addr)
	if err != nil {
		log.Fatalf("could not listen: %s", err)
	}

	log.Printf("listening addr=%s", listener.Addr())
	http.Serve(listener, nil)
}

func FileReadingHandler(w http.ResponseWriter, r *http.Request) {
	time.Sleep(*timeout)

	file, _, err := r.FormFile("f")
	if err != nil {
		log.Printf("error reading file: %s", err)
	}

	n, err := ioutil.ReadAll(file)
	log.Printf("n=%d err=%q", n, err)
}