JSON编码为零值的嵌入式json.Marshaler恐慌，您会认为这是一个错误吗？

Question

json.NewEncoder(os.Stdout).Encode(struct {
    M json.Marshaler
}{})

panics:

panic: interface conversion: interface is nil, not json.Marshaler 

https://play.golang.org/p/Vur0JJq6SF

I'd like to explicitly state for my interface that it should marshal itself:

type thing interface {
    json.Marshaler
    Type() string
    ....
}

would you consider this a bug? If no, why not?