diff --git a/sys/netinet/ip_output.c b/sys/netinet/ip_output.c
--- a/sys/netinet/ip_output.c
+++ b/sys/netinet/ip_output.c
@@ -1067,7 +1067,7 @@
 			csum = 0xffff;
 	} else {
 		cklen = ntohs(ip->ip_len);
-		csum = in_cksum_skip(m, cklen, offset);
+		csum = in_cksum_skip(m, cklen + iph_offset, offset);
 	}
 	offset += m->m_pkthdr.csum_data;	/* checksum offset */