json Unmarshal error

I'm getting an error of:

json.Unmarshal undefined (type interface {} has no field or method Unmarshal)

trying to convert a json byte slice into the generic interface{} type. I'm reading the docs for encoding/json and they give an example that shows this is valid. What gives?

package main

import (
    "encoding/json"
    "fmt"
    "io/ioutil"
)

func main() {

    var json interface{}
    data, _ := ioutil.ReadFile("testMusic.json")
    json.Unmarshal(data, &json)
    m := json.(map[string]interface{})
    fmt.Printf("%+v", m)

}