could not calculate upgrade 12.04 to 14.04

[Kevin]

when upgrading to 14.04LTS upgrade tool error:

Could not calculate the upgrade
An unresolvable problem occurred while calculating the upgrade.
 This can be caused by:
 * Upgrading to a pre-release version of Ubuntu
 * Running the current pre-release version of Ubuntu
* Unofficial software packages not provided by Ubuntu
If none of this applies, then please report this bug using the command 'ubuntu-bug ubuntu-release-upgrader-core' in a terminal.

That's what I did.

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: update-manager 1:0.156.14.13
ProcVersionSignature: Ubuntu 3.2.0-58.88-generic-pae 3.2.53
Uname: Linux 3.2.0-58-generic-pae i686
ApportVersion: 2.0.1-0ubuntu17.6
Architecture: i386
Date: Tue May 6 11:51:06 2014
GsettingsChanges:
 com.ubuntu.update-manager first-run false
 com.ubuntu.update-manager launch-time 1399399828
 com.ubuntu.update-manager window-height 500
 com.ubuntu.update-manager window-width 600
InstallationMedia: Ubuntu 12.04 LTS "Precise Pangolin" - Release i386 (20120423)
MarkForUpload: True
PackageArchitecture: all
ProcEnviron:
 LANGUAGE=en_CA:en
 TERM=xterm
 PATH=(custom, no user)
 LANG=en_CA.UTF-8
 SHELL=/bin/bash
SourcePackage: update-manager
Symptom: ubuntu-release-upgrader-core
UpgradeStatus: Upgraded to precise on 2014-05-06 (0 days ago)
VarLogDistupgradeTermlog: