Index: sys/netpfil/ipfw/nat64/nat64_translate.c
===================================================================
--- sys/netpfil/ipfw/nat64/nat64_translate.c
+++ sys/netpfil/ipfw/nat64/nat64_translate.c
@@ -1234,6 +1234,12 @@
 		ip6.ip6_hlim -= IPTTLDEC;
 	ip6.ip6_plen = htons(plen);
 	ip6.ip6_nxt = (proto == IPPROTO_ICMP) ? IPPROTO_ICMPV6: proto;
+
+	/* Handle delayed checksums for locally originated packets */
+	if (m->m_pkthdr.csum_flags & CSUM_DELAY_DATA) {
+		in_delayed_cksum(m);
+		m->m_pkthdr.csum_flags &= ~CSUM_DELAY_DATA;
+	}
 	/* Convert checksums. */
 	switch (proto) {
 	case IPPROTO_TCP:
@@ -1604,6 +1610,12 @@
 		return (NAT64RETURN);
 	}
 	nat64_init_ip4hdr(ip6, frag, plen, proto, &ip);
+
+	/* Handle delayed checksums for locally originated packets */
+	if (m->m_pkthdr.csum_flags & CSUM_DELAY_DATA_IPV6) {
+		in6_delayed_cksum(m, plen, hlen);
+		m->m_pkthdr.csum_flags &= ~CSUM_DELAY_DATA_IPV6;
+	}
 	/* Convert checksums. */
 	switch (proto) {
 	case IPPROTO_TCP: