diff --git a/product/ERP5/Document/Rule.py b/product/ERP5/Document/Rule.py
index 6170213c6efe2bd8da8acc7dc16767b26fd17b40..00d5603835a1c5816b332f7ada2406d07cafa1bd 100644
--- a/product/ERP5/Document/Rule.py
+++ b/product/ERP5/Document/Rule.py
@@ -383,5 +383,7 @@ class Rule(XMLObject, Predicate):
       else:
         # movement not modifiable, we can decide to create a compensation
         # with negative quantity
-        raise NotImplementedError("Not Implemented")
+        raise NotImplementedError(
+                "Can not create a compensation movement for %s" % \
+                movement.getRelativeUrl())
     return (add_list, modify_dict, delete_list)