The LookupError lets one think that the database is missing an object, but in reality the select criterias are wrong

plone/formwidget/contenttree/source.py

@@ -133,7 +133,7 @@ class PathSource(object):
         if brain is None:
             return self._placeholderTerm(value)
         if not self.isBrainSelectable(brain):
-            raise LookupError(value)
+            raise LookupError('Value "%s" does not match criteria for field' % str(value))
         return self.getTermByBrain(brain)
 
     # Query API - used to locate content, e.g. in non-JS mode