--- ./in_mcast.c	2009-09-03 12:09:18.000000000 +0300
+++ /usr/src/sys/netinet/in_mcast.c	2009-09-03 11:55:20.000000000 +0300
@@ -33,7 +33,7 @@
  */
 
 #include <sys/cdefs.h>
-__FBSDID("$FreeBSD$");
+__FBSDID("$FreeBSD: src/sys/netinet/in_mcast.c,v 1.27 2009/08/01 19:26:27 rwatson Exp $");
 
 #include <sys/param.h>
 #include <sys/systm.h>
@@ -1899,6 +1899,9 @@
 			ssa->sin.sin_addr = mreqs.imr_sourceaddr;
 		}
 
+		if (!IN_MULTICAST(ntohl(gsa->sin.sin_addr.s_addr)))
+			return (EINVAL);
+
 		ifp = inp_lookup_mcast_ifp(inp, &gsa->sin,
 		    mreqs.imr_interface);
 		CTR3(KTR_IGMPV3, "%s: imr_interface = %s, ifp = %p",
@@ -1936,6 +1939,9 @@
 			ssa->sin.sin_port = 0;
 		}
 
+		if (!IN_MULTICAST(ntohl(gsa->sin.sin_addr.s_addr)))
+			return (EINVAL);
+
 		if (gsr.gsr_interface == 0 || V_if_index < gsr.gsr_interface)
 			return (EADDRNOTAVAIL);
 		ifp = ifnet_byindex(gsr.gsr_interface);
@@ -1948,9 +1954,6 @@
 		break;
 	}
 
-	if (!IN_MULTICAST(ntohl(gsa->sin.sin_addr.s_addr)))
-		return (EINVAL);
-
 	if (ifp == NULL || (ifp->if_flags & IFF_MULTICAST) == 0)
 		return (EADDRNOTAVAIL);