diff --git a/tests/sys/netinet/ip_reass_test.c b/tests/sys/netinet/ip_reass_test.c
--- a/tests/sys/netinet/ip_reass_test.c
+++ b/tests/sys/netinet/ip_reass_test.c
@@ -56,23 +56,20 @@
 };
 
 static void
-update_cksum(struct ip *ip)
+update_cksum(void *ip)
 {
 	size_t i;
+	uint16_t *cksump;
 	uint32_t cksum;
-	uint8_t  *cksump;
-	uint16_t tmp;
 
-	ip->ip_sum = 0;
-	cksump = (char *)ip;
-	for (cksum = 0, i = 0; i < sizeof(*ip) / sizeof(uint16_t); i++) {
-		tmp = *cksump++;
-		tmp = tmp << 8 | *cksump++;
-		cksum += ntohs(tmp);
-	}
-	cksum = (cksum >> 16) + (cksum & 0xffff);
-	cksum = ~(cksum + (cksum >> 16));
-	ip->ip_sum = htons((uint16_t)cksum);
+	((struct ip *)ip)->ip_sum = 0;
+	cksump = (uint16_t *)ip;
+	cksum = 0;
+	for (i = 0; i < sizeof(struct ip) / sizeof(uint16_t); i++)
+		cksum += *cksump++;
+	while ((cksum >> 16) != 0)
+		cksum = (cksum & 0xffff) + (cksum >> 16);
+	((struct ip *)ip)->ip_sum = (uint16_t)(~cksum);
 }
 
 static struct lopacket *