diff --git a/main.go b/main.go
index e01bb61..92dd9b4 100644
--- a/main.go
+++ b/main.go
@@ -149,7 +149,7 @@ func main() {
 		os.Exit(1)
 	}
 	log.Println("Source port:", sourcePort)
-	if flag.Lookup("u") != nil {
+	if isUdp {
 		log.Println("Protocol:", "udp")
 		isUdp = true
 	} else {