在类型切换中使用strconv.FormatFloat（）遇到麻烦

I'm simply trying to use a type switch for handling times, float32s and float64s. It's working fine for times and float64s, but strconv.FormatFloat(val, 'f', -1, 32) keeps telling me that I can't use type float32 as type float64. I don't see how that could be happening, so I must be missing something or misunderstanding how I'm supposed to be calling FormatFloat() for float32s.

func main() {
    fmt.Println(test(rand.Float32()))
    fmt.Println(test(rand.Float64()))
}

func test(val interface{}) string {
    switch val := val.(type) {
        case time.Time:
            return fmt.Sprintf("%s", val)
        case float64:
            return strconv.FormatFloat(val, 'f', -1, 64)
        case float32:
            return strconv.FormatFloat(val, 'f', -1, 32) //here's the error
        default:
            return "Type not supported!"
    }
}

Error:

cannot use val (type float32) as type float64 in argument to strconv.FormatFloat

The first argument to FormatFloat needs to be a float64, and you are passing a float32. The easiest way to fix this is to simply cast the 32bit float to a 64bit float.

case float32:
    return strconv.FormatFloat(float64(val), 'f', -1, 32)

http://play.golang.org/p/jBPaQ-jMBT