diff --git a/protocol/json/server_request.go b/protocol/json/server_request.go
index db9d221..ed294d4 100644
--- a/protocol/json/server_request.go
+++ b/protocol/json/server_request.go
@@ -167,7 +167,7 @@ func (src *ServerRequestCodec) WriteError(w io.Writer, status int, err error) er
 
 func (src *ServerRequestCodec) writeServerResponse(w io.Writer, res *serverResponse) error {
 	// ID is null for notifications and they don't have a response.
-	if req.ID != nil {
+	if res.ID != nil {
 		encoder := json.NewEncoder(src.codec.Encode(w))
 		// Not sure in which case will this happen. But seems harmless.
 		if err := encoder.Encode(res); nil != err {