Index: if_ethersubr.c
===================================================================
--- if_ethersubr.c	(revision 332091)
+++ if_ethersubr.c	(working copy)
@@ -40,6 +40,7 @@
 #include <sys/systm.h>
 #include <sys/bus.h>
 #include <sys/eventhandler.h>
+#include <sys/kdb.h>
 #include <sys/kernel.h>
 #include <sys/lock.h>
 #include <sys/malloc.h>
@@ -48,6 +49,7 @@
 #include <sys/random.h>
 #include <sys/socket.h>
 #include <sys/sockio.h>
+#include <sys/syslog.h>
 #include <sys/sysctl.h>
 #include <sys/uuid.h>
 
@@ -444,6 +446,12 @@ ether_output_frame(struct ifnet *ifp, struct mbuf
 {
 	int i;
 
+	/* KASSERT(curvnet != 0, ("curvnet is NULL")); */
+	log(LOG_WARNING, "%s: %s: curvnet %p\n",
+	                    __func__, ifp->if_xname, curvnet);
+#ifdef KDB
+	kdb_backtrace();
+#endif
 	if (PFIL_HOOKED(&V_link_pfil_hook)) {
 		i = pfil_run_hooks(&V_link_pfil_hook, &m, ifp, PFIL_OUT, NULL);