Marillat's signature invalid

[Max <relf@unn.ac.ru>]

Hello!

"apt-get update" reports that following:

W: GPG error: http://spello.sscnet.ucla.edu sid Release: The following signatures were invalid: BADSIG 07DC563D1F41B907 Christian Marillat <marillat@debian.org>
W: You may want to run apt-get update to correct these problems

Re-run of "apt-get update" does not help.
What's wrong and how to fix the problem?

Thanks,
Max