nvdl and xinclude question

I'm xincluding a bibliography in my chapter. Validation (nvdl) gives me an error that a bibliography is not allowed. But when I physically replace the xinclude with the actual included file, validation passes. Here are the details:

mybook_mychapter.xml has this structure:
<chapter xmlns="http://docbook.org/ns/docbook"
                  xmlns:xl="http://www.w3.org/1999/xlink"
                  xml:id="mybook.mychapter"
                  remap="Chapter" version="5.0">
<info><title>my chapter</title></info>
<section xml:id="a0000000002" remap="Headi">
   <para> lots of stuff in here</para>
</section>
<xi:include xmlns:xi="http://www.w3.org/2001/XInclude" href="mybook_mychapter_bib.xml"/>
</chapter>

I give this command:
java -Dorg.apache.xerces.xni.parser.XMLParserConfiguration=org.apache.xerces.parsers.XIncludeParserConfiguration -jar onvdl.jar docbook.nvdl mybook_mychapter.xml

and get this response:
error: element "bibliography" not allowed in this context

The contents of mybook_mychapter_bib.xml is:
<bibliography>
  <title>References</title>
  <bibliomixed xml:id="kemp_o_57"/>
  <bibliomixed xml:id="falc_d_96"/>
  <bibliomixed xml:id="crow_j_70"/>
</bibliography>

When I replace the xinclude with the actual lines from the mybook_mychapter_bib.xml, the validation passes.

any ideas on what I'm doing wrong?
thanks,
--Tim Arnold

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