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.