Singing a key with a subkey

[Brian Mearns]

I've exported a crippled version of my private keyset for use at
work...I did not include the primary/master key in the export, only a
signing subkey and an encryption subkey. Now I've imported them on a
different system and want to sign a co-workers key with the subkey,
but gpg complains that:

gpg: secret key parts are not available
gpg: signing failed: general error

I'm able to sign files using the subkey (on the same system), so I'm
not clear why I wouldn't be able to sign a key with it as well. Is
there something I'm missing, is there a way around this, or is there
something fundamental about this limitation?

I'm using gpg 1.4.9 on Windows XP.

Thanks for any help
-Brian

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