After running for a while, "Install Updates" button does not function

This only seems to happen when update-manager has been left running for a while. In ~/.xsession-errors I see:

Traceback (most recent call last):
  File "/usr/lib/python2.6/dist-packages/UpdateManager/UpdateManager.py", line 586, in on_button_install_clicked
    self.cache.checkFreeSpace()
  File "/usr/lib/python2.6/dist-packages/DistUpgrade/DistUpgradeCache.py", line 1104, in checkFreeSpace
    for (dir, size) in [(archivedir, self.requiredDownload),
  File "/usr/lib/python2.6/dist-packages/UpdateManager/Core/MyCache.py", line 88, in requiredDownload
    pm.GetArchives(fetcher, self._list, self._records)
SystemError: E:I wasn't able to locate file for the base-files package. This might mean you need to manually fix this package.

If I instead click 'Check', 'Install Updates' will work - even if I cancel out of the Check.