dkim signing is failing (according to spamassassin) and I need details to solve it

I am using qmail and libdkimtest to sign outgoing emails. The setup is almost exactly as per this tutorial (http://patchlog.com/security/qmail-and-dkim/).

Emails I send out have a signature added, and it looks fine to me however, spamassassin reports:

X-Spam-Report: 
    *  0.1 DKIM_SIGNED Message has a DKIM or DK signature, not necessarily
    *      valid
    *  0.0 T_DKIM_INVALID DKIM-Signature header exists but is not valid

I've added the DNS records exactly as many tutorials suggest. I've tested the DNS with http://www.sendmail.org/dkim/checker and it doesn't report any problems, and displays the public key correctly.

I would like to get more detailed errors in order to figure out why spamassassin says it isn't valid.

Thanks.