Index: sys/kern/vfs_lookup.c
===================================================================
--- sys/kern/vfs_lookup.c	(revision 354857)
+++ sys/kern/vfs_lookup.c	(working copy)
@@ -326,6 +326,11 @@
 	MPASS(ndp->ni_startdir == NULL || ndp->ni_startdir->v_type == VDIR ||
 	    ndp->ni_startdir->v_type == VBAD);
 	fdp = p->p_fd;
+
+	/* Bail out if root file system is not mounted */
+	if (__predict_false(fdp->fd_rdir == NULL))
+		return (ENXIO);
+
 	TAILQ_INIT(&ndp->ni_cap_tracker);
 	ndp->ni_lcf = 0;