Erro no update

Olá pessoas,
ao dar update no meu ubuntu 12.4 retorna esse erro:

vanexa@vanexa-desktop:~$ sudo apt-get update
[sudo] password for vanexa:
Ign http://br.archive.ubuntu.com precise InRelease
Ign http://br.archive.ubuntu.com precise-security InRelease
Ign http://br.archive.ubuntu.com precise-updates InRelease
Ign http://br.archive.ubuntu.com precise-backports InRelease
Obter:1 http://packages.medibuntu.org precise InRelease
Ign http://extras.ubuntu.com precise InRelease
Atingido http://br.archive.ubuntu.com precise Release.gpg
Atingido http://br.archive.ubuntu.com precise-security Release.gpg
Obter:2 http://br.archive.ubuntu.com precise-updates Release.gpg [198 B]
Obter:3 http://packages.medibuntu.org precise Release.gpg
Atingido http://extras.ubuntu.com precise Release.gpg
Obter:4 http://br.archive.ubuntu.com precise-backports Release.gpg [198 B]
Atingido http://br.archive.ubuntu.com precise Release
Atingido http://extras.ubuntu.com precise Release
Atingido http://br.archive.ubuntu.com precise-security Release
Obter:5 http://packages.medibuntu.org precise Release
Ign http://packages.medibuntu.org precise Release
E: Erro GPG: http://packages.medibuntu.org precise Release: As seguintes assinaturas eram inválidas: NODATA 1 NODATA 2
vanexa@vanexa-desktop:~$

Já tentei vários tutoriais mas nada corrige isso.. Alguém pode me ajudar a resolver?
Agradeço desde já!!!!!!