Can't verify signature made by PGP

[Carson Hewitt]

Hello,

Someone must have run into this before:

I can't verify a signature made by PGP using GnuPG 1.4.10 (on mac os 10.6.1).

The idea is to send a detached signature to the PGP Timestamping service at pgp 
_AT_ stamper.itconsult.co.uk, which signs it.

gpg --verify emits a warning saying that only the first signature in the reply 
will be verified (the other one is the one I made), then the verification fails 
(bad signature).

Is giving up using the service for that scenario the only thing to do ?

 -- Carson