Lucid to Precise: purging GDM fails

Setting up gdm (3.0.4-0ubuntu15) ...

$ LC_ALL=C sudo dpkg -P --force-depends gdm
(Reading database ... 118811 files and directories currently installed.)
Removing gdm ...
stop: Unknown instance:
Purging configuration files for gdm ...
Removing user `gdm' ...
Warning: group `gdm' has no more members.
userdel: user gdm is currently logged in
/usr/sbin/deluser: `/usr/sbin/userdel gdm' returned error code 8. Exiting.
Could not remove gdm user.
/usr/sbin/delgroup: `gdm' still has `gdm' as their primary group!
Could not remove gdm group.
userdel: user gdm is currently logged in
/usr/sbin/deluser: `/usr/sbin/userdel gdm' returned error code 8. Exiting.
/usr/sbin/delgroup: `gdm' still has `gdm' as their primary group!
dpkg: error processing gdm (--purge):
 subprocess installed post-removal script returned error exit status 128
Processing triggers for ureadahead ...
Processing triggers for gconf2 ...
Processing triggers for hicolor-icon-theme ...
Errors were encountered while processing:
 gdm

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: gdm (not installed)
ProcVersionSignature: Ubuntu 3.2.0-16.25-generic-pae 3.2.6
Uname: Linux 3.2.0-16-generic-pae i686
ApportVersion: 1.91-0ubuntu1
Architecture: i386
Date: Thu Feb 16 20:09:50 2012
ProcEnviron:
 LANGUAGE=fi:en
 PATH=(custom, user)
 LANG=fi_FI.UTF-8
 SHELL=/bin/bash
SourcePackage: gdm
UpgradeStatus: Upgraded to precise on 2010-05-01 (656 days ago)