diff -r fb932bcfd803 -r b40c2bbb8da5 lemon/network_simplex.h
--- a/lemon/network_simplex.h	Sat Sep 04 23:58:03 2010 +0200
+++ b/lemon/network_simplex.h	Mon Aug 19 22:35:54 2013 +0200
@@ -929,7 +929,7 @@
       for (NodeIt n(_graph); n != INVALID; ++n, ++i) {
         _node_id[n] = i;
       }
-      if (_arc_mixing) {
+      if (_arc_mixing && _node_num > 1) {
         // Store the arcs in a mixed order
         const int skip = std::max(_arc_num / _node_num, 3);
         int i = 0, j = 0;