diff -r 1bafdbd2fc46 -r ee9bac10f58e lemon/cost_scaling.h
--- a/lemon/cost_scaling.h	Sat Mar 20 11:03:12 2010 +0100
+++ b/lemon/cost_scaling.h	Sat Mar 16 16:20:41 2013 +0100
@@ -761,6 +761,10 @@
       }
       if (_sum_supply > 0) return INFEASIBLE;
 
+      // Check lower and upper bounds
+      LEMON_DEBUG(checkBoundMaps(),
+          "Upper bounds must be greater or equal to the lower bounds");
+
 
       // Initialize vectors
       for (int i = 0; i != _res_node_num; ++i) {
@@ -897,6 +901,15 @@
 
       return OPTIMAL;
     }
+    
+    // Check if the upper bound is greater or equal to the lower bound
+    // on each arc.
+    bool checkBoundMaps() {
+      for (int j = 0; j != _res_arc_num; ++j) {
+        if (_upper[j] < _lower[j]) return false;
+      }
+      return true;
+    }
 
     // Execute the algorithm and transform the results
     void start(Method method) {