When 'Update System' is invoked manually or automatically in Gnome, it fails with the following error:

Error Type: <type 'exceptions.AttributeError'>
Error Value: 'NoneType' object has no attribute 'timestamp'
  File : /usr/share/PackageKit/helpers/yum/refresh-cache.py , line 18, in <module>
    backend.refresh_cache()
  File : /usr/share/PackageKit/helpers/yum/yumBackend.py , line 876, in refresh_cache
    self.yumbase.repos.populateSack(which=[repo.id],mdtype='metadata',cacheonly=1)
  File : /usr/lib/python2.5/site-packages/yum/repos.py , line 251, in populateSack
    sack.populate(repo, mdtype, callback, cacheonly)
  File : /usr/lib/python2.5/site-packages/yum/yumRepo.py , line 159, in populate
    if self._check_db_version(repo, mydbtype):
  File : /usr/lib/python2.5/site-packages/yum/yumRepo.py , line 216, in _check_db_version
    return repo._check_db_version(mdtype)
  File : /usr/lib/python2.5/site-packages/yum/yumRepo.py , line 958, in _check_db_version
    repoXML = self.repoXML
  File : /usr/lib/python2.5/site-packages/yum/yumRepo.py , line 1116, in <lambda>
    repoXML = property(fget=lambda self: self._getRepoXML(),
  File : /usr/lib/python2.5/site-packages/yum/yumRepo.py , line 1108, in _getRepoXML
    self._loadRepoXML(text=self)
  File : /usr/lib/python2.5/site-packages/yum/yumRepo.py , line 1098, in _loadRepoXML
    return self._groupLoadRepoXML(text, ["primary"])
  File : /usr/lib/python2.5/site-packages/yum/yumRepo.py , line 1082, in _groupLoadRepoXML
    if self._commonLoadRepoXML(text):
  File : /usr/lib/python2.5/site-packages/yum/yumRepo.py , line 951, in _commonLoadRepoXML
    if not self._groupCheckDataMDNewer():
  File : /usr/lib/python2.5/site-packages/yum/yumRepo.py , line 910, in _groupCheckDataMDNewer
    if old_repo_XML.timestamp > self.repoXML.timestamp: