diff --git a/sys/netinet6/icmp6.c b/sys/netinet6/icmp6.c
--- a/sys/netinet6/icmp6.c
+++ b/sys/netinet6/icmp6.c
@@ -2569,6 +2569,7 @@
 			/* pad if easy enough, truncate if not */
 			if (8 - extra <= M_TRAILINGSPACE(m0)) {
 				/* pad */
+				bzero(mtod(m0, caddr_t) + m0->m_len, 8 - extra);
 				m0->m_len += (8 - extra);
 				m0->m_pkthdr.len += (8 - extra);
 			} else {