docbook-xsl: Title page customization: "no namespace bound to prefix xhtml"

I am attempting to customize xhtml title page output from DocBook
sources.  I process the following title page template:

titlepage_templates.xml
---
<t:templates xmlns:t="http://nwalsh.com/docbook/xsl/template/1.0"
             xmlns:param="http://nwalsh.com/docbook/xsl/template/1.0/param"
             xmlns:xhtml="http://www.w3.org/1999/xhtml"
             xmlns:xsl="http://www.w3.org/1999/XSL/Transform">

  <t:titlepage t:element="article" t:wrapper="xhtml:div" class="titlepage">
    <t:titlepage-content t:side="recto">
      <title/>
      <subtitle/>

      <abstract/>

      <orgname/>

      <legalnotice/>

      <revhistory/>
    </t:titlepage-content>

    <t:titlepage-content t:side="verso"/>

    <t:titlepage-separator>
      <xhtml:hr/>
    </t:titlepage-separator>

    <t:titlepage-before t:side="recto"/>

    <t:titlepage-before t:side="verso"/>
  </t:titlepage>
</t:templates>
---

With the following command line:

$ xsltproc http://docbook.sourceforge.net/release/xsl/1.68.1/template/titlepage.xsl titlepage_templates.xml

And I get a sequence of errors like:

runtime error: file http://docbook.sourceforge.net/release/xsl/1.68.1/template/titlepage.xsl line 628 element element
xsl:element : no namespace bound to prefix xhtml

But I've declared the xhtml namespace prefix in the root element of my
template file.  I'm confused; can someone help to determine what's wrong
here?

Thanks,

    John L. Clark

P.S. If this is a QNames-in-attribute-values problem, I do believe I
might break something.  Oh, wait, something's already broken.