Problem with tables and roundtrip to WordML

I'm trying to convert a very simple docbook file to WordML using the roundtrip XSL stylesheets.  I'm having trouble with tables: all cell contents are missing after the conversion.

Here is my input document:

<?xml version="1.0" encoding="UTF-8"?><!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN" "http://www.oasis-open.org/doc\

book/xml/4.5/docbookx.dtd"><article lang="en">

<table frame='all'><title>Sample Table</title>

  <tgroup cols="2">    <colspec colname='c1'/><colspec colname='c2'/>

    <thead><row><entry>a1</entry><entry>a2</entry></row></thead>    <tbody><row><entry>b1</entry><entry>b2</entry></row></tbody>

  </tgroup></table>

</article>

This renders ok in HTML with:

xsltproc --path .../docbook-xsl/html -o test.html \

  docbook.xsl test.xml

but with this: 

xsltproc --path .../docbook-xsl/roundtrip -o test.wml \
  --stringparam wordml.template template.xml \

  dbk2wordml.xsl test2.xml

the cells in the resulting table are all empty, i.e. the resulting WordML looks like:

            <w:tc>
              <w:tcPr>
                <w:tcW w:w="" w:type="dxa"/>

              </w:tcPr>
              <w:p/>
            </w:tc>

I get the same result with either xsltproc or saxon.  I am using docbook-xsl-1.76.1.

My long term goal is to be able to programmatically generate reports as templated, styled Word documents from asciidoc source files, with docbook as an intermediate format, in a linux environment.  I need relatively few features: headers, body text, simple bullet lists, PNG images, and reasonably professional-looking tables (where that means I need control over column alignment and borders).  

-- Dave