golang fails to parse json for reflection created object

I try to write simple message protocol in go and i've encountered a problem. I have a lot of message types and i want to have a dictionary like this to manipulate with messages:

var dict map[reflect.Type]int = map[reflect.Type]int{
    reflect.TypeOf(DataMessage{}):          1000,
    reflect.TypeOf(TextMessage{}):          1001,
    //....
}

func GetMessageTypeId(value interface{}) int {
    if id, ok := dict[reflect.TypeOf(value)]; ok {
        return id
    } else {
        return -1
    }
}

func GetValueByTypeId(typeId int) interface{} {
    for typeDec, id := range dict {
        if id == typeId {
            return reflect.New(typeDec).Interface()
        }
    }
    fmt.Println("Unknown message type", typeId)
    return nil
}

It works fine, but when i instantiate message with GetValueByTypeId and try to unmarshall json into it - i receive map[string]interface instead of my message. I've made simple example to reproduce the problem:

http://ift.tt/1hxeCE0