&& in the email

Hey,

An email like this is said by firegpg to have invalid signature (The first signature of this email is not valid. (invalid signature))

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

monica&&mat
-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.4.6 (GNU/Linux)

iD8DBQFIhu4hIvBv0k5esR4RAu8HAJ9WE3NZmb358KYDvJZN5kQgFhMAgwCbBISR
BtnU5MpEdYrs/Y/OEw4xpRY=
=hl/7
-----END PGP SIGNATURE-----

but

$ echo 'monica&&mat' | gpg --clearsign 

You need a passphrase to unlock the secret key for
user: "Mateusz Blaszczyk <xxxx@xxxxxxxxxxxx.xx>"
1024-bit DSA key, ID 4E5EB11E, created 2008-06-01

gpg: gpg-agent is not available in this session
-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

monica&&mat
-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.4.6 (GNU/Linux)

iD8DBQFIhuwhIvBv0k5esR4RAuOpAKCyCpd5+EO413cNNmPET0FVlDZ+fQCffChU
sO2B0fJXHIS5ab+W2AXIWFA=
=MM++
-----END PGP SIGNATURE-----

$ cat tt | gpg
monica&&mat
gpg: Signature made Wed 23 Jul 2008 09:30:25 IST using DSA key ID 4E5EB11E
gpg: Good signature from "Mateusz Blaszczyk <xxxx@xxxxxxxx.xx>"
gpg: WARNING: This key is not certified with a trusted signature!
gpg:          There is no indication that the signature belongs to the owner.
Primary key fingerprint: 3DC2 73E3 B342 6717 9634  2699 22F0 6FD2 4E5E B11E

What's the story?

using Ubuntu 8.04 + gpg (GnuPG) 1.4.6 + Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9) Gecko/2008061015 Firefox/3.0

Best Regards,

-mat