vscode python refactor failed

When i tried to rename myset in the following code, i saw the following message.

class NumberGenerator:
    def __init__(self):
        self.__myset__ = set()

    def uncount(self, i):
        self.__myset__.add(i)

Refactor failed. module 'rope.base.ast' has no attribute 'Str'
[<FrameSummary file refactor.py, line 294 in watch>, <FrameSummary file refactor.py, line 274 in _process_request>, <FrameSummary file refactor.py, line 206 in _rename>, ...

It's so long, if you want to see i will show it all. Is this code wrong? But It works. Why Refactor doesn't work?