Please answer these questions before submitting your issue. Thanks!

1. What version of Go are you using (go version)?
   1.6
2. What operating system and processor architecture are you using (go env)?
   darwin/amd6
3. What did you do?
   https://play.golang.org/p/MsVP9u-We5
4. What did you expect to see?
   {"M":null}
   https://golang.org/pkg/encoding/json/#Marshal says "A nil interface value encodes as the null JSON object."
5. What did you see instead?
   panic: interface conversion: interface is nil, not json.Marshaler