diff --git a/sys/netgraph/ng_nat.c b/sys/netgraph/ng_nat.c
--- a/sys/netgraph/ng_nat.c
+++ b/sys/netgraph/ng_nat.c
@@ -884,6 +884,10 @@
 			    ip_len - (ip->ip_hl << 2)));
 
 			if ((m->m_pkthdr.csum_flags & CSUM_TCP) == 0) {
+				m = mb_unmapped_to_ext(m);
+				if (m == NULL) {
+					return (ENOMEM);
+				}
 				m->m_pkthdr.csum_data = offsetof(struct tcphdr,
 				    th_sum);
 				in_delayed_cksum(m);