diff --git a/sys/netinet/tcp_subr.c b/sys/netinet/tcp_subr.c
index d4c552f03cc..e38a2a7bf45 100644
--- a/sys/netinet/tcp_subr.c
+++ b/sys/netinet/tcp_subr.c
@@ -420,6 +420,7 @@ tcp_respond(tp, ipgen, th, m, ack, seq, flags)
 	nth->th_urp = 0;
 #ifdef INET6
 	if (isipv6) {
+		nth->th_sum = 0;
 		nth->th_sum = in6_cksum(m, IPPROTO_TCP,
 					sizeof(struct ip6_hdr),
 					tlen - sizeof(struct ip6_hdr));
diff --git a/sys/netinet/tcp_timewait.c b/sys/netinet/tcp_timewait.c
index d4c552f03cc..e38a2a7bf45 100644
--- a/sys/netinet/tcp_timewait.c
+++ b/sys/netinet/tcp_timewait.c
@@ -420,6 +420,7 @@ tcp_respond(tp, ipgen, th, m, ack, seq, flags)
 	nth->th_urp = 0;
 #ifdef INET6
 	if (isipv6) {
+		nth->th_sum = 0;
 		nth->th_sum = in6_cksum(m, IPPROTO_TCP,
 					sizeof(struct ip6_hdr),
 					tlen - sizeof(struct ip6_hdr));