diff --git a/sys/netinet/tcp_subr.c b/sys/netinet/tcp_subr.c
--- a/sys/netinet/tcp_subr.c
+++ b/sys/netinet/tcp_subr.c
@@ -1809,6 +1809,8 @@
 		mtu = ip_next_mtu(oldmtu, 0);
 	}
 
+	if (mtu < sizeof(struct tcpiphdr))
+		mtu = sizeof(struct tcpiphdr);
 	if (isipv6)
 		rt = tcp_rtlookup6(&inp->inp_inc);
 	else