--- a/macof.c
+++ b/macof.c
@@ -134,7 +134,7 @@ main(int argc, char *argv[])
 		libnet_build_tcp(sport, dport, seq, 0, TH_SYN, 512,
 				 0, 0, LIBNET_TCP_H, NULL, 0, l, 0);
 		
-		libnet_build_ipv4(LIBNET_TCP_H, 0,
+		libnet_build_ipv4(LIBNET_IPV4_H + LIBNET_TCP_H, 0,
 				  libnet_get_prand(LIBNET_PRu16), 0, 64,
 				  IPPROTO_TCP, 0, src, dst, NULL, 0, l, 0);