no info to calculate a trust probability

[Nick THOMPSON]

Hi,

I have search high and low for an answer to this, but to no avail: I am
using Evolution as my mail tool and am trying to encrypt a mail using a
key that somebody sent me, which this header

-----BEGIN PGP PUBLIC KEY BLOCK-----
Version: PGP 7.1.1

The key looks like after importing to gpg:

pub  1024D/A55C22FA 1999-11-23 Xxxxxx Xxxx <xxxx.xxxx@xxxxxxx.co.uk>
sub  2048g/29622D17 1999-11-23

(The name and address are changed to protect the innocent)

gpg says:
# gpg --verbose --no-secmem-warning --no-greeting --yes --batch --armor
-r xxxx.xxxx --encrypt file
gpg: using secondary key 29622D17 instead of primary key A55C22FA
gpg: 29622D17: no info to calculate a trust probability
gpg: no valid addressees
gpg: file: encryption failed: no such user id

I have signed the key. If I use gpg interactively I can force the
encryption, but Evolution only uses gpg in batch mode. Any idea how I
can get enough info into this key to make it work?

Thanks,
Nick