merge() fails for one to one relationships marked load_on_pending=True and delete_orphan

AttributeError: 'symbol' object has no attribute '_sa_instance_state'

# orm/unitofwork.py
def track_cascade_events(descriptor, prop):
    """Establish event listeners on object attributes which handle
    cascade-on-set/append.

    """
    ...
    ...
    ...
    def set_(state, newvalue, oldvalue, initiator):
        # process "save_update" cascade rules for when an instance 
        # is attached to another instance
        if oldvalue is newvalue:
            return newvalue

        sess = session._state_session(state)
        if sess:
            prop = state.manager.mapper._props[key](key)
            if newvalue is not None:
                newvalue_state = attributes.instance_state(newvalue)
                if prop.cascade.save_update and \
                    (prop.cascade_backrefs or key == initiator.key) and \
                    not sess._contains_state(newvalue_state):
                    sess._save_or_update_state(newvalue_state)

            if oldvalue is not None and prop.cascade.delete_orphan:
                oldvalue_state = attributes.instance_state(oldvalue)  # <=== oldvalue is PASSIVE_NO_RESULT

                if oldvalue_state in sess._new and \
                    prop.mapper._is_orphan(oldvalue_state):
                    sess.expunge(oldvalue)
        return newvalue
    ...
    ...
    event.listen(descriptor, 'set', set_, raw=True, retval=True)

history from http://groups.google.com/group/sqlalchemy/browse_thread/thread/51b134cd70af6fe7

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