Ellipsis not working for anything but strings

[codesoap]

Suppose I have this program:

package main

import (
	`fmt`
	`github.com/docopt/docopt-go`
)

func main() {
	opts, _ := docopt.ParseDoc(`
		Usage:
		    mytool --number=<num>...`)

	var conf struct {
		Number []int
	}
	if err := opts.Bind(&conf); err != nil {
		panic(err)
	}

	fmt.Printf("%v\n", conf)
}

When executing go run mytool.go I get this error:

panic: value of "--number" is not assignable to "Number" field

goroutine 1 [running]:
main.main()
        /<path_to_script>/mytool.go:17 +0x190
exit status 2

If I change []int to []string everything works.