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It happens because pgp 6 does not follow the OpenPGP Message Format (rfc2440). In particular, when generating x.509 signatures, it does not include the mandatory issuer subpacket at all (as one of the first subpackets, or any other), as described in section 5.2.4.1:
An implementation SHOULD put the two mandatory subpackets, creation time and issuer, as the first subpackets in the subpacket list, simply to make it easier for the implementer to find them.
Because I have limited time to put together a new ``official'' patch, here is an unofficial one. This patch is relative to the official patch 2.
Last updated 27 June 2000.