Index: sys/netinet6/in6_mcast.c
===================================================================
--- sys/netinet6/in6_mcast.c	(revision 342073)
+++ sys/netinet6/in6_mcast.c	(working copy)
@@ -632,6 +632,8 @@
 		    i6mm_chain, imm_tmp) {
 			if (inm == imm->i6mm_maddr) {
 				LIST_REMOVE(imm, i6mm_chain);
+				inm->in6m_refcount--;
+				MPASS(inm->in6m_refcount > 0);
 				free(imm, M_IP6MADDR);
 			}
 		}
Index: sys/netinet6/mld6.c
===================================================================
--- sys/netinet6/mld6.c	(revision 342073)
+++ sys/netinet6/mld6.c	(working copy)
@@ -2004,6 +2004,7 @@
 		if (mli->mli_version == MLD_VERSION_2 &&
 		    inm->in6m_state == MLD_LEAVING_MEMBER) {
 			inm->in6m_refcount--;
+			MPASS(inm->in6m_refcount > 0);
 		}
 		inm->in6m_state = MLD_REPORTING_MEMBER;