GnuPG and PGP 2.6: unusable public key

[Tommi Vainikainen]

I got public key from a friend.  That key was generated with some
version of PGP 2.6.  To import that key I had to use
--allow-non-selfsigned-uid, but now the key is in my public keyring.

When I'm trying to encrypt message to him, I got very short and
nothing explaining error message "unusable public key".  So what is
the reason key is unusable, how do I know this reason?  (This is a bug
in GnuPG, would I say.)  Even --verbose didn't bother to explain more.

bash:~$ gpg --verbose --rfc1991 --cipher-algo idea --compress-algo  1 -a --encrypt -r bob
gpg: bob: skipped: unusable public key
gpg: [stdin]: encryption failed: unusable public key
bash:~$ 

So what should I do to be able to encrypt messages to my friend?  (And
btw, I've idea-extension in my gpg.)

-- 
Tommi Vainikainen