Polymorphic relationships fail in a concrete table inheritance scenario

(original reporter: ged) See attached test case (which is a modified version of test/orm/inheritance/concrete.py).
Here is the traceback.
  File "/home/ged/concrete_with_relations.py", line 188, in test_relation_2
    res = session.query(Company).one().engineers
  File "/home/ged/devel/sqlalchemy/trunk/lib/sqlalchemy/orm/query.py", line 921, in one
    ret = list(self[0:2](0:2))
  File "/home/ged/devel/sqlalchemy/trunk/lib/sqlalchemy/orm/query.py", line 985, in iterate_instances
    rows = [process[0](process[0)(context, row) for row in fetch]
  File "/home/ged/devel/sqlalchemy/trunk/lib/sqlalchemy/orm/query.py", line 1543, in main
    extension=context.extension, only_load_props=context.only_load_props, refresh_instance=context.refresh_instance
  File "/home/ged/devel/sqlalchemy/trunk/lib/sqlalchemy/orm/mapper.py", line 1435, in _instance
    self.populate_instance(context, instance, row, only_load_props=only_load_props, instancekey=identitykey, isnew=isnew)
  File "/home/ged/devel/sqlalchemy/trunk/lib/sqlalchemy/orm/mapper.py", line 1500, in populate_instance
    selectcontext.exec_with_path(self, key, populator, instance, row, ispostselect=ispostselect, isnew=isnew, **flags)
  File "/home/ged/devel/sqlalchemy/trunk/lib/sqlalchemy/orm/query.py", line 1664, in exec_with_path
    return fn(*args, **kwargs)
  File "/home/ged/devel/sqlalchemy/trunk/lib/sqlalchemy/orm/strategies.py", line 622, in execute
    self.mapper._instance(selectcontext, decorated_row, result_list)
  File "/home/ged/devel/sqlalchemy/trunk/lib/sqlalchemy/orm/mapper.py", line 1359, in _instance
    return mapper._instance(context, row, result=result, polymorphic_from=self)
  File "/home/ged/devel/sqlalchemy/trunk/lib/sqlalchemy/orm/mapper.py", line 1435, in _instance
    self.populate_instance(context, instance, row, only_load_props=only_load_props, instancekey=identitykey, isnew=isnew)
  File "/home/ged/devel/sqlalchemy/trunk/lib/sqlalchemy/orm/mapper.py", line 1484, in populate_instance
    poly_select_loader = self._get_poly_select_loader(selectcontext, row)
  File "/home/ged/devel/sqlalchemy/trunk/lib/sqlalchemy/orm/mapper.py", line 1522, in _get_poly_select_loader
    cond, param_names = self._deferred_inheritance_condition(hosted_mapper, needs_tables)
  File "/home/ged/devel/sqlalchemy/trunk/lib/sqlalchemy/orm/mapper.py", line 1587, in _deferred_inheritance_condition
    allconds.append(visitors.traverse(mapper.inherit_condition, clone=True, visit_binary=visit_binary))
  File "/home/ged/devel/sqlalchemy/trunk/lib/sqlalchemy/sql/visitors.py", line 181, in traverse
    return vis.traverse(clause, clone=clone)
  File "/home/ged/devel/sqlalchemy/trunk/lib/sqlalchemy/sql/visitors.py", line 73, in traverse
    return self._cloned_traversal(obj)
  File "/home/ged/devel/sqlalchemy/trunk/lib/sqlalchemy/sql/visitors.py", line 108, in _cloned_traversal
    return self._cloned_traversal_impl(obj, util.Set(stop_on), {}, _clone_toplevel=True)
  File "/home/ged/devel/sqlalchemy/trunk/lib/sqlalchemy/sql/visitors.py", line 115, in _cloned_traversal_impl
    elem = self._clone_element(elem, stop_on, cloned)
  File "/home/ged/devel/sqlalchemy/trunk/lib/sqlalchemy/sql/visitors.py", line 101, in _clone_element
    cloned[elem](elem) = elem._clone()
AttributeError: 'NoneType' object has no attribute '_clone'
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