Unable to convert newtypes of slices to the primitive slice

[SOF3]

package main

type Message []byte

func main() {
        var x Message = []byte("abc")
        println([]byte(x)) // line 7
}

(dlv) c main.go:7
(dlv) p []byte(x)
Command failed: can not convert "x" to []uint8

However, surprisingly, p string(x) prints "abc" correctly.