Traceback (most recent call last):
  File "/usr/share/software-center/softwarecenter/view/appdetailsview_gtk.py", line 641, in mark_changes
    installed = self.view.cache[addon].installed
  File "/usr/share/software-center/softwarecenter/apt/aptcache.py", line 115, in __getitem__
    return self._cache[key]
  File "/usr/lib/python2.7/dist-packages/apt/cache.py", line 168, in __getitem__
    raise KeyError('The cache has no package named %r' % key)
KeyError: "The cache has no package named 'cantor-backend-sage'"