Index: print-udp.c
===================================================================
--- print-udp.c	(revision 261187)
+++ print-udp.c	(working copy)
@@ -546,9 +546,11 @@
 #ifdef INET6
 		else if (IP_V(ip) == 6 && ip6->ip6_plen) {
 			/* for IPv6, UDP checksum is mandatory */
-			if (TTEST2(cp[0], length)) {
+			udp_sum = EXTRACT_16BITS(&up->uh_sum);
+			if (udp_sum == 0) {
+				(void)printf("[invalid cksum 0] ");
+			} else if (TTEST2(cp[0], length)) {
 				sum = udp6_cksum(ip6, up, length + sizeof(struct udphdr));
-				udp_sum = EXTRACT_16BITS(&up->uh_sum);
 
 	                        if (sum != 0) {
         	                        (void)printf("[bad udp cksum 0x%04x -> 0x%04x!] ",