apt - missing trustedkeys.gpg to verify source package

[Alois Mahdal <Alois.Mahdal.1-ndmail@zxcvb.cz>]

Hello,

when downloading source packages, I see error regarding
trustedkeys.gpg:

    $ apt-get source mousepad
    [...]
    Fetched 454 kB in 0s (1,050 kB/s)
    gpgv: keyblock resource `/home/me/.gnupg/trustedkeys.gpg':
    file open error gpgv: Signature made Sat 30 Jun 2012
    05:49:12 PM CEST using RSA key ID 71EF0BA8 gpgv: Can't
    check signature: public key not found dpkg-source: warning:
    failed to verify signature on ./mousepad_0.2.16-6.dsc
    [...]


What is wrong?  I tried to search for the missing file via
packages.debian.org/search but no package seems to contain the
file.

My sources.list contains deb and deb-src for Wheezy and sid,
Pinning being set to 650 and 600.  Similarly named file
~/.gnupg/trustdb.gpg ("GPG key trust database version 3" in
`file`'s words) exists.

TIA,
aL.

-- 
Alois Mahdal