Can't create workitem of type 'Defect'

Hi Di Xu@dixudx
It raise exception: NotFound: Not itemtype's name is Defect in when I try to create one, but it can fetch the type attribute as 'defect' from an existing defect.
Can you take a look?
my RTC version is 6.0.2,
here is the code:
myClient = RTCClient(url, username, password, ends_with_jazz=False)
myClient.createWorkitem(item_type="Defect",
title='defect demo',
description=None,
projectarea_id='xxx',
projectarea_name='xxxx',
template=None,
copied_from=19,
keep=True)