Apt key error

[Jean-Eric]

Hi,
Since installing Breezy (I never had that under Hoary), I have this 
error message sometimes:
Reading package lists... Done
W: GPG error: http://ch.archive.ubuntu.com breezy-updates Release: The 
following signatures were invalid: BADSIG 40976EAF437D05B5 Ubuntu 
Archive Automatic Signing Key <ftpmaster at ubuntu.com>
W: You may want to run apt-get update to correct these problems

Do you have and idea what happens?
And is there a workaround or some fix?

Thanks.
-jec