third-party software sources not loading

Traceback (most recent call last):
  File "/usr/lib/pymodules/python2.6/ubuntutweak/mainwindow.py", line 436, in setup_notebook
    page = module()
  File "/usr/lib/pymodules/python2.6/ubuntutweak/modules/thirdsoft.py", line 647, in __init__
    if os.getenv('LANG').startswith('zh_CN'):
AttributeError: 'NoneType' object has no attribute 'startswith'

Is the error I get when I try to open third-party sources window in ubuntu-tweak 0.5.0 karmic ppa.