使用docopt输入问题，然后转到

I'm writing some code in go and i want to pass some inputs with docopt. With the first version, I pass via command line only a string, and everything works fine. With the second one, I want to pass two strings and it doesn't work and I get this error:

panic: runtime error: slice bounds out of range [:5] with length 0

goroutine 1 [running]:
main.main()
    /home/user/go/src/github.com/keygen/cmd/keygen/main.go:45 +0xa56

It seems the first string passed has lenght 0.

I think the problem is in the line where I call the Default.Parser, but I can't find the solution. Could you help me?

This is the code not working:

package main

import (
    "fmt"
    "github.com/docopt/docopt.go"
    "github.com/keygen"
    "os"
    "strconv"
)

const usage = `Keygen

Usage:
  keygen (-m|-n|-c) [-l <length>] <serialStart> <serialStop>
  keygen -h | --help

Options:
  -l <length>     Output key length [default: 10].
  -h --help       Show this screen.`

func main() {
    var count keygen.Count
    var Serial string
    var args struct {
        SerialStart  string `docopt:"<serialStart>"`
        SerialStop   string `docopt:"<serialStop>"`
        Length       int    `docopt:"-l"`
        First        bool   `docopt:"-m"`
        Second       bool   `docopt:"-n"`
        Third        bool   `docopt:"-c"`
    }

    opts, err := docopt.DefaultParser.ParseArgs(usage, os.Args[1:], "")
    if err != nil {
        return
    }

        prefix := args.SerialStart[:5]
..........................................

This is the code working:

package main

import (
    "fmt"
    "github.com/docopt/docopt.go"
    "github.com/keygen"
    "os"
)

const usage = `Keygen

Usage:
  keygen (-m|-n|-c) [-l <length>] <serial>
  keygen -h | --help

Options:
  -l <length>     Output key length [default: 10].
  -h --help       Show this screen.`

func main() {
    var count keygen.Count
        var Serial string
    var args struct {
        Serial       string `docopt:"<serial>"`
        Length       int    `docopt:"-l"`
        First        bool   `docopt:"-m"`
        Second       bool   `docopt:"-n"`
        Third        bool   `docopt:"-c"`
    }

    opts, err := docopt.DefaultParser.ParseArgs(usage, os.Args[1:], "")
        if err != nil {
        return
    }

        prefix := args.Serial[:5]
........................................