DOCBOOK: URL with CGI information

Hello,

Sorry if this issue is covered elsewhere or has been covered before. I have
been unable to find any clues.

The problem I am having is best illustrated with a quick example:

<!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook V3.1//EN">

<article>
    <para>
    <ulink
url="/scripts/script.php?button_new=yes&category=test">Sript</ulink>.
    </para>
</article>

Which produces the error:
/usr/local/bin/jade:test.sgml:5:51:E: general entity "category" not defined
and no default entity

Can anyone give me any help as to how to deal with this?

Thanks for any help

Kevin