Fix Error response prior to being able to parse the Request.ID

handler.go

@@ -87,7 +87,13 @@ func processRequest(methods MethodMap, rawReq json.RawMessage) Response {
 	var req safeRequest
 	if err := unmarshalStrict(rawReq, &req); err != nil ||
 		!req.IsValid() {
-		return newErrorResponse(nil, InvalidRequest)
+		// Since the request was not valid for some reason, we cannot
+		// assume anything about the ID or whether it was a
+		// Notification or not. Thus we must return an Error Response.
+		// In order to ensure the handler doesn't omit the Response by
+		// confusing it with a Notification, we must populate the ID
+		// field with the JSON null value.
+		return newErrorResponse(json.RawMessage("null"), InvalidRequest)
 	}
 	// Clear null params before calling the method.
 	if string(req.Params) == "null" {