diff --git a/calma/CalmaWriteZ.c b/calma/CalmaWriteZ.c
index 557541b4..b60f0b6d 100644
--- a/calma/CalmaWriteZ.c
+++ b/calma/CalmaWriteZ.c
@@ -1830,8 +1830,11 @@ calmaProcessBoundaryZ(
 	    calmaOutI4Z(lbref->lb_start.p_y * calmaPaintScale, f);
 	    chkcount++;
 	}
-	calmaOutI4Z(listtop->lb_start.p_x * calmaPaintScale, f);
-	calmaOutI4Z(listtop->lb_start.p_y * calmaPaintScale, f);
+	if (listtop != NULL)
+	{
+	    calmaOutI4Z(listtop->lb_start.p_x * calmaPaintScale, f);
+	    calmaOutI4Z(listtop->lb_start.p_y * calmaPaintScale, f);
+	}
 
 	if (chkcount != bounds->bt_points)
 	    TxError("Points recorded=%d;  Points written=%d\n",