Erron on uptating to Pangolin Alpha2

[Ruben Tomas]

Can't install 'ubuntu-desktop'

It was impossible to install a required package. Please report this as a bug using 'ubuntu-bug update-manager' in a terminal.

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: update-manager 1:0.156.2
ProcVersionSignature: Ubuntu 3.2.0-12.21-generic 3.2.2
Uname: Linux 3.2.0-12-generic x86_64
ApportVersion: 1.91-0ubuntu1
Architecture: amd64
Date: Thu Feb 2 22:49:45 2012
GsettingsChanges:
 com.ubuntu.update-manager first-run false
 com.ubuntu.update-manager launch-time 1328219324
 com.ubuntu.update-manager show-details true
 com.ubuntu.update-manager window-height 740
 com.ubuntu.update-manager window-width 600
InstallationMedia: Ubuntu 10.04.2 LTS "Lucid Lynx" - Release amd64 (20110211.1)
PackageArchitecture: all
SourcePackage: update-manager
UpgradeStatus: Upgraded to precise on 2012-02-02 (0 days ago)

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Antec (info-janmob)]

I would update from Precise, alpha1 to Precise, alpha2 update can not find a shell, I've only had one installed and it is Unity, so the update is interrupted