OTA of rtm/14.09-proposed/krillin from 240 to 241 fails with systemimage.gpg.SignatureError

OTA from 240 to 241 fails with the following error message in /var/log/system-image/client.log

[systemimage] Feb 20 09:09:37 2015 (21576) check_for_update failed
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/systemimage/api.py", line 104, in check_for_update
    self._state.run_until('download_files')
  File "/usr/lib/python3/dist-packages/systemimage/state.py", line 175, in run_until
    step()
  File "/usr/lib/python3/dist-packages/systemimage/state.py", line 294, in _get_channel
    ctx.validate(asc_path, channels_path)
  File "/usr/lib/python3/dist-packages/systemimage/gpg.py", line 216, in validate
    self.keyring_paths, self.blacklist_path)
systemimage.gpg.SignatureError:
    sig path : b23d5b2a14027f6775f4012123f5e8c1
               /tmp/system-image-bk33lz3z/channels.json.asc
    data path: 2e42b9826ba73fb2b4a79e6769012c17
               /tmp/system-image-bk33lz3z/channels.json
    keyrings : ['45d56eb2cbc1394a6c3dd3e51d7a256c']
               ['/var/lib/system-image/keyrings/image-signing.tar.xz']
    blacklist: 573804b9f5149eaf253c8cbaa7ab1b83 /var/lib/system-image/blacklist.tar.xz

[systemimage] Feb 20 09:33:26 2015 (21576) test and acquire checking lock
[systemimage] Feb 20 09:33:26 2015 (21576) checking lock not acquired

And 241 is never visible for upgrade.

ProblemType: Bug
DistroRelease: Ubuntu RTM 14.09
Package: system-image-common 2.5.1-0ubuntu1~rtm2
Uname: Linux 3.4.67 armv7l
ApportVersion: 2.14.7-0ubuntu8
Architecture: armhf
Date: Fri Feb 20 09:52:14 2015
InstallationDate: Installed on 2015-02-18 (1 days ago)
InstallationMedia: Ubuntu Utopic Unicorn (development branch) - armhf (20150218-184321)
PackageArchitecture: all
SourcePackage: system-image
UpgradeStatus: No upgrade log present (probably fresh install)