diff --git a/sys/netinet6/in6_mcast.c b/sys/netinet6/in6_mcast.c
--- a/sys/netinet6/in6_mcast.c
+++ b/sys/netinet6/in6_mcast.c
@@ -2030,7 +2030,7 @@
 			 * state at t1, because allocation of a filter
 			 * is atomic with allocation of a membership.
 			 */
-			error = EINVAL;
+			error = EADDRINUSE;
 			goto out_in6p_locked;
 		}
 	}