docopt.go weird error message

using docopt.go to refactor an old project and minimalise code the program looks like this

package main

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

const Version = `2.0`
const Usage = `
Usage:
	serve [--port] <dir>
	serve help | --help
	serve --version 

Options:
	-p, --port       port for the sever to listen on
	-h, --help       display help information
	-v, --version    display Version
`

func check(err error) {
	if err != nil {
		panic(err)
	}
}

func main() {
	args, err := docopt.Parse(Usage, nil, true, Version, false)
	check(err)

	port := args["[--port]"].(string)

	fmt.Println(args)
	fmt.Println(port)v
}

however when I run the program go run ./serve.go help expecting the help message i get this

panic: interface conversion: interface is nil, not string

goroutine 1 [running]:
main.main()
	/Users/jburns/Development/Gopath/src/github.com/nyumal/serve/serve.go:31 +0x148

goroutine 2 [runnable]:
runtime.forcegchelper()
	/usr/local/Cellar/go/1.4.1/libexec/src/runtime/proc.go:90
runtime.goexit()
	/usr/local/Cellar/go/1.4.1/libexec/src/runtime/asm_amd64.s:2232 +0x1

goroutine 3 [runnable]:
runtime.bgsweep()
	/usr/local/Cellar/go/1.4.1/libexec/src/runtime/mgc0.go:82
runtime.goexit()
	/usr/local/Cellar/go/1.4.1/libexec/src/runtime/asm_amd64.s:2232 +0x1

goroutine 4 [runnable]:
runtime.runfinq()
	/usr/local/Cellar/go/1.4.1/libexec/src/runtime/malloc.go:712
runtime.goexit()
	/usr/local/Cellar/go/1.4.1/libexec/src/runtime/asm_amd64.s:2232 +0x1
exit status 2

and running go run ./serve.go --port 5000 it returns the same thing however running go run ./serve.go --port 5000 . return

Usage:
	serve [--port] <dir>
	serve help | --help
	serve --version
exit status 1

where did i go wrong?

You need to declare an argument for port:

const Usage = `
Usage:
    serve [--port=<arg>] <dir>
    serve help | --help
    serve --version 

Options:
    -p, --port=<arg> port for the sever to listen on
    -h, --help       display help information
    -v, --version    display Version

Use the two value type assertion to handle the case where port is not set:

port, ok := args["--port"].(string)
if ok {
   // port is set
}

Also, remove the "[]" from around the map key.