Error using PGP 5 generated key

[Johan Wevers]

Hello,

I have created a DSA/ElGamal key with pgp 5.0i for Linux (to ensure I
remained compatible with PGP users) and try to use it with GnuPG 1.0.0.
Encrypting to myself works, but signing does not:

vulcan:~/tmp> gpg -r johanw@vulcan.xs4all.nl -es testfile

You need a passphrase to unlock the secret key for
user: "Johan Wevers <johanw@vulcan.xs4all.nl>"
3072-bit ELG-E key, ID 1B240FEE, created 1999-10-26 (main key ID 624B3B3E)

gpg: this is a PGP generated ElGamal key which is NOT secure for signatures!
gpg: no valid signators
gpg: testfile: sign+encrypt failed: no such user id

gpg --sign-key results in an error:

Command> sign
Really sign all user IDs? y
gpg: this is a PGP generated ElGamal key which is NOT secure for signatures!
gpg: no valid signators

What is going on? Is this key really insecure (and if so, why?), or are
there some obscure trust settings that are incorrect? PGP 5 seems able to
use it for signatures.

-- 
ir. J.C.A. Wevers         //  Physics and science fiction site:
johanw@vulcan.xs4all.nl   //  http://www.xs4all.nl/~johanw/index.html
PGP public keys at http://www.xs4all.nl/~johanw/pgpkeys.html