Index: sys/netinet/in_mcast.c
===================================================================
--- sys/netinet/in_mcast.c
+++ sys/netinet/in_mcast.c
@@ -1865,8 +1865,7 @@
  * specific physical links in the networking stack, or which need
  * to join link-scope groups before IPv4 addresses are configured.
  *
- * If inp is non-NULL, use this socket's current FIB number for any
- * required FIB lookup.
+ * Use this socket's current FIB number for any required FIB lookup.
  * If ina is INADDR_ANY, look up the group address in the unicast FIB,
  * and use its ifp; usually, this points to the default next-hop.
  *
@@ -1887,8 +1886,8 @@
 	struct rm_priotracker in_ifa_tracker;
 	struct ifnet *ifp;
 	struct nhop_object *nh;
-	uint32_t fibnum;
 
+	KASSERT(inp != NULL, ("%s: inp must not be NULL", __func__));
 	KASSERT(gsin->sin_family == AF_INET, ("%s: not AF_INET", __func__));
 	KASSERT(IN_MULTICAST(ntohl(gsin->sin_addr.s_addr)),
 	    ("%s: not multicast", __func__));
@@ -1901,8 +1900,7 @@
 			if_ref(ifp);
 		IN_IFADDR_RUNLOCK(&in_ifa_tracker);
 	} else {
-		fibnum = inp ? inp->inp_inc.inc_fibnum : RT_DEFAULT_FIB;
-		nh = fib4_lookup(fibnum, gsin->sin_addr, 0, NHR_NONE, 0);
+		nh = fib4_lookup(inp->inp_inc.inc_fibnum, gsin->sin_addr, 0, NHR_NONE, 0);
 		if (nh != NULL) {
 			ifp = nh->nh_ifp;
 			if_ref(ifp);