DOCBOOK: xref not working

Hi folks,

I'm using RedHat Linux 7.1, and docbook2pdf doesn't seem to process
<xref> elements properly. it gives the following error:

-------------------------------------------------------------------
jade:/usr/share/sgml/docbook/dsssl-stylesheets-1.59/print/dblink.dsl:179:1:E:
XRef LinkEnd to missing ID 'chapter1'
-------------------------------------------------------------------

when running on the following file:

-------------------------------------------------------------------
<?xml version="1.0"?>

<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook V3.1//EN"[]>

<book>

<bookinfo>
<title>Test Book</title>

<authorgroup><author><firstname>Zack</firstname><surname>Brown</surname>
<affiliation><address><email></email></address>
</affiliation></author></authorgroup>

<copyright><year>2002</year><holder>me</holder></copyright>

<legalnotice>
<para>This document may be copied freely.</para>
</legalnotice>
</bookinfo>
<chapter id="chapter1">
<title>ch1</title>
<para>test paragraph</para>
</chapter>
<chapter>
<title>ch2</title>
<para>another test para. See <xref linkend="chapter1"/></para>
</chapter>
</book>
-------------------------------------------------------------------

Does anyone know how I can get this working?

Many thanks in advance,
Zack


-- 
Zack Brown