While running the demo, the client shows "transport: missing content-type field"

[MikeChongCan]

I'm testing the demo following the README. However, it shows remote read conn closed: rpc error: code = Unknown desc = : HTTP status code 521; transport: missing content-type field

I used grpcurl and it shows the same:

grpcurl -proto ./pkg/proto/gun.proto  -vv foo.bar.com:443 GunService/Tun

Resolved method descriptor:
rpc Tun ( stream .Hunk ) returns ( stream .Hunk );

Request metadata to send:
(empty)

Response trailers received:
(empty)
Sent 0 requests and received 0 responses
ERROR:
  Code: Unknown
  Message: : HTTP status code 521; transport: missing content-type field