diff --git a/server.go b/server.go
index 81580d1e5..b04d370f6 100644
--- a/server.go
+++ b/server.go
@@ -226,6 +226,7 @@ type Server struct {
 	// If NotifyStartedFunc is set it is called once the server has started listening.
 	NotifyStartedFunc func()
 	// DecorateReader is optional, allows customization of the process that reads raw DNS messages.
+	// The decorated reader must not mutate the data read from the conn.
 	DecorateReader DecorateReader
 	// DecorateWriter is optional, allows customization of the process that writes raw DNS messages.
 	DecorateWriter DecorateWriter