Comment 1 for bug 1913476

Traceback (most recent call last):
  File "/usr/bin/update-manager", line 118, in <module>
    app.start_update()
  File "/usr/lib/python3/dist-packages/UpdateManager/UpdateManager.py", line 220, in start_update
    self.start_available()
  File "/usr/lib/python3/dist-packages/UpdateManager/UpdateManager.py", line 238, in start_available
    self.refresh_cache()
  File "/usr/lib/python3/dist-packages/UpdateManager/UpdateManager.py", line 435, in refresh_cache
    duplicate_packages=self.duplicate_packages)
  File "/usr/lib/python3/dist-packages/UpdateManager/Core/UpdateList.py", line 531, in update
    eventloop_callback)
  File "/usr/lib/python3/dist-packages/UpdateManager/Core/UpdateList.py", line 444, in _make_groups
    if pkg in cache:
  File "/usr/lib/python3/dist-packages/apt/cache.py", line 350, in __contains__
    return self.__is_real_pkg(self._cache[key])
TypeError: Expected a string or a pair of strings

This is due to apt.package.Package objects being added to a list of strings.