如何在另一个地图界面中添加地图界面？

Here I have message a map interface. There are 2 keys in it key1, key2. Key2 has 2 keys k2, k3. I want to add another key in it. So I did this.

message := map[string]interface{}{
    "key1": map[string]string{
        "k1" : "",
    },
    "key2": map[string]interface{}{
        "k2": "",
        "k3" : map[string]interface{}{
            "kk1"       : "", 
        },
    },
}
k := map[string]interface{}{
        "kk2"   : "",
}
message["key2"]["k4"] = k

b, err := json.Marshal(message)
if err != nil {
    fmt.Println(err)
}
fmt.Println(string(b))

But I am getting error as:

message["key2"]["k4"] (type interface {} does not support indexing)

Assert the interface{} value to a map[string]interface{} and index that.

message["key2"].(map[string]interface{})["k4"] = k