How to convert map[string]interface{} to map[string]string

I'm trying to convert a map[string]interface{} to map[string]string and whenever I try I get a panic:

package main

import (
	"encoding/json"
)

func main() {
	var doc interface{}
	js := `{ "id": "123", "links": { "self": "/app/123" } }`
	json.Unmarshal([]byte(js), &doc)
	jsonObj, _ := doc.(map[string]interface{})
	myFunc_convert(jsonObj)
}

func myFunc_convert(js map[string]interface{}) {
  links := js["links"].(map[string]string)
  links["hi"] = "foo"
}

func myFunc_noConvert(js map[string]interface{}) {
  links := js["links"]
  links["hi"] = "foo"
}

Gives me the following error if I call myFunc_convert:

interface conversion: interface {} is map[string]interface {}, not map[string]string

If I call myFunc_noConvert I get this error:

invalid operation: links["hi"] (type interface {} does not support indexing)

I know ahead of time that links will be a map[string]string and I need to be able to add items to it. Is there a way to do this that I'm missing?

I thought about creating a new map of type map[string]string, then looping through everything in links and converting it to type string and putting it in the new map, but that seems clunky.

Is there a better way to do this?

Here you go:

package main

import (
	"encoding/json"
	"fmt"
	"log"
)

func main() {
	var doc interface{}
	js := `{ "id": "123", "links": { "self": "/app/123" } }`
	if err := json.Unmarshal([]byte(js), &doc); err != nil {
		log.Fatal(err)
	}
	jsonObj, _ := doc.(map[string]interface{})
	links := myFunc(jsonObj)
	links["hi"] = "foo"
	fmt.Printf("%+v\n", links)
}

func myFunc(js map[string]interface{}) map[string]string {
	result := map[string]string{}
	links, ok := js["links"].(map[string]interface{})
	if !ok {
		return result
	}
	for k := range links {
		if v, ok := links[k].(string); ok {
			result[k] = v
		}
	}
	return result
}