Upgrading ubuntu to version 12.04 failed

A Fatal Error Occured

Please report this as a bug (if you haven't already) and include the files /var/log/dist-upgrade/main.log and /var/log/dist-upgrade/apt.log in your report. The upgrade has aborted.
Your original sources.list was saved in /etc/apt/sources.list.distUpgrade.

Traceback (most recent call last):

  File "/tmp/update-manager-dFF9DU/precise", line 7, in <module>
    sys.exit(main())

  File "/tmp/update-manager-dFF9DU/DistUpgradeMain.py", line 226, in main
    if app.run():

  File "/tmp/update-manager-dFF9DU/DistUpgradeController.py", line 1721, in run
    return self.fullUpgrade()

  File "/tmp/update-manager-dFF9DU/DistUpgradeController.py", line 1703, in fullUpgrade
    self.doPostUpgrade()

  File "/tmp/update-manager-dFF9DU/DistUpgradeController.py", line 1152, in doPostUpgrade
    now_obsolete = self.cache._getObsoletesPkgs()

  File "/tmp/update-manager-dFF9DU/DistUpgradeCache.py", line 985, in _getObsoletesPkgs
    for pkg in self:

  File "/usr/lib/python2.7/dist-packages/apt/cache.py", line 189, in __iter__
    yield self[pkgname]

  File "/usr/lib/python2.7/dist-packages/apt/cache.py", line 174, in __getitem__
    pkg = self._weakref[key] = Package(self, self._cache[key])

KeyError: '0ad-dbg:i386'