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diff --git a/lib/dotgen/conc.c b/lib/dotgen/conc.c
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--- a/lib/dotgen/conc.c
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+++ b/lib/dotgen/conc.c
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@@ -159,7 +159,11 @@ static void rebuild_vlists(graph_t * g)
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     for (r = GD_minrank(g); r <= GD_maxrank(g); r++) {
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 	lead = GD_rankleader(g)[r];
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-	if (GD_rank(dot_root(g))[r].v[ND_order(lead)] != lead) {
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+	if (lead == NULL) {
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+		agerr(AGERR, "rebuiltd_vlists: lead is null for rank %d\n", r);
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+		longjmp(jbuf, 1);
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+	}
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+	else if (GD_rank(dot_root(g))[r].v[ND_order(lead)] != lead) {
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 	    agerr(AGERR, "rebuiltd_vlists: rank lead %s not in order %d of rank %d\n", 
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 		agnameof(lead), ND_order(lead), r);
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 	    longjmp(jbuf, 1);