--- a/sys/netinet6/ip6_mroute.c
+++ b/sys/netinet6/ip6_mroute.c
@@ -423,7 +423,7 @@
 	    so->so_proto->pr_protocol != IPPROTO_ICMPV6)
 		return EOPNOTSUPP;
 
-	if (!m || (m->m_len != sizeof(int *)))
+	if (!m || (m->m_len < sizeof(int)))
 		return ENOPROTOOPT;
 
 	v = mtod(m, int *);