diff --git a/lib/dotgen/conc.c b/lib/dotgen/conc.c

--- a/lib/dotgen/conc.c

+++ b/lib/dotgen/conc.c

@@ -159,7 +159,11 @@ static void rebuild_vlists(graph_t * g)

     for (r = GD_minrank(g); r <= GD_maxrank(g); r++) {

 	lead = GD_rankleader(g)[r];

-	if (GD_rank(dot_root(g))[r].v[ND_order(lead)] != lead) {

+	if (lead == NULL) {

+		agerr(AGERR, "rebuiltd_vlists: lead is null for rank %d\n", r);

+		longjmp(jbuf, 1);

+	}

+	else if (GD_rank(dot_root(g))[r].v[ND_order(lead)] != lead) {

 	    agerr(AGERR, "rebuiltd_vlists: rank lead %s not in order %d of rank %d\n", 

 		agnameof(lead), ND_order(lead), r);

 	    longjmp(jbuf, 1);