Can't use underlying type of string inside map of string: "cannot use <xxx> (type <yyy>) as type string in map index"

I have underlying type of string: type Capability string.

I wanted to use it as a string inside map of strings but I am getting an error:

cannot use cap (type Capability) as type string in map index

This is my code:

package main

import (
    "fmt"
)


type Capability string
var caps_list = map[string]int {
    "HOME" : 1,
}

func main() {

    var cap Capability // string
    cap = "HOME"
    fmt.Print(string(caps_list[cap]))
}  

Why it doesn't accept it ? it is a string after all.

You can try my code here:
https://play.golang.org/p/r-h9Hu8_eoM