update-manager complains about a missing 'ubuntu-minimal' package, while it is installed

[Alvin]

Upgrade according to: https://help.ubuntu.com/community/PreciseUpgrades/Kubuntu/10.04LTS
update-manager complains about a missing package: ubuntu-minimal:
"After your package information was updated the essential package 'ubuntu-minimal' can not be found anymore so a bug reporting process is being started."

But:
~> aptitude show ubuntu-minimal
Package: ubuntu-minimal
State: installed

ProblemType: Bug
DistroRelease: Ubuntu 10.04
Package: update-manager (not installed)
ProcVersionSignature: Ubuntu 2.6.32-39.86-generic 2.6.32.56+drm33.22
Uname: Linux 2.6.32-39-generic x86_64
NonfreeKernelModules: nvidia
Architecture: amd64
Date: Tue Mar 27 15:36:54 2012
ProcEnviron:
 LANGUAGE=
 PATH=(custom, user)
 LANG=C
 SHELL=/bin/tcsh
SourcePackage: update-manager
VarLogDistupgradeAptlog:
 Log time: 2012-03-27 15:35:30.800839
 Log time: 2012-03-27 15:36:38.888291
 Log time: 2012-03-27 15:36:49.352201
 Log time: 2012-03-27 15:36:58.078082
VarLogDistupgradeTermlog:

[Alvin (alvind)]

Switched mirrors. Upgrading now.

[Brian Murray (brian-murray)]

This is because you were using a mirror not know to update-manager. It is possible to use third party mirrors if you set an enivornmental variable.
        if (self.config.getWithDefault("Sources","AllowThirdParty",False) or
            "RELEASE_UPRADER_ALLOW_THIRD_PARTY" in os.environ):
            logging.warning("mirror check skipped, *overriden* via config")
            mirror_check=False

However, as you've switched mirrors I am marking this as invalid.