Failed to retrieve download mirrors

[cell]

hello and welcome (my 1st post here).
now a few days later,

Code:

apt-get update

gives me something like:

Code:

W: A error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: mirror://apt.xbian.org wheezy Release: The following signatures were invalid: NODATA 1 NODATA 2
W: Failed to fetch mirror://apt.xbian.org/mirror.txt/dists/wheezy/Release
W: Some index files failed to download. They have been ignored, or old ones used instead.

any idea? just downloaded beta 1.1
tx.