diff --git a/sys/netinet/ip_output.c b/sys/netinet/ip_output.c
index 4a93d1c05b6..7533e099c00 100644
--- a/sys/netinet/ip_output.c
+++ b/sys/netinet/ip_output.c
@@ -1083,8 +1083,8 @@ in_delayed_cksum_o(struct mbuf *m, uint16_t iph_offset)
 		if (csum == 0)
 			csum = 0xffff;
 	} else {
-		cklen = ntohs(ip->ip_len);
-		csum = in_cksum_skip(m, cklen, offset);
+		cklen = ntohs(ip->ip_len) - (ip->ip_hl << 2);
+		csum = in_cksum_skip(m, cklen + offset, offset);
 	}
 	offset += m->m_pkthdr.csum_data;	/* checksum offset */