Problems validating file with xinclude content

Good morning:

I have the following file that runs multiple XIncluded content. 

<?xml version="1.0" encoding="UTF-8"?>
<book version="5.0" xml:id="ruby-on-rails-3.2-en" xml:lang="en"

      xmlns="http://docbook.org/ns/docbook"
      xmlns:xlink="http://www.w3.org/1999/xlink"
      xmlns:xi="http://www.w3.org/2001/XInclude"

      xmlns:svg="http://www.w3.org/2000/svg"
      xmlns:m="http://www.w3.org/1998/Math/MathML"
      xmlns:html="http://www.w3.org/1999/xhtml"

      xmlns:db="http://docbook.org/ns/docbook">
  <info>
    <title>Ruby on Rails 3.2 - Step by Step</title>

    <abstract>

      <para>This is a step by step book to learn how to code Ruby 1.9 and use Ruby on Rails 3.2 to create modern web applications.</para>
    </abstract>

    <edition>1.0</edition>

    <author>
      <personname><firstname>Some</firstname><surname>Author</surname></personname>

      <email></email>

      <affiliation>
        <orgname>AMOOMA GmbH</orgname>
      </affiliation>
    </author>

    <pubdate>2012</pubdate>
  </info>

  <dedication>

    <para>For my family.</para>
  </dedication>

  <preface xml:id="preface">
    <title>Preface</title>

    <para>Content for the preface goes here...</para>

  </preface>

  <xi:include href="">
  <xi:include href="">
  <xi:include href="">

  <xi:include href="">
  <xi:include href="">
  <!-- There is more xi:included content. This was trimmed for the email -->
  <index xml:id="stichwortverzeichnis"/>

</book>

When I try to validate with Jing, I get the following errors:

     [java] 
/Users/carlos.araya/code/ruby-book/en/docbook/index.xml:68:60: error: 
element "xi:include" not allowed here; expected the element end-tag or 
element "acknowledgements", "appendix", "article", "bibliography", 
"chapter", "colophon", "dedication", "glossary", "index", "part", 
"preface", "reference" or "toc"
     [java] 
/Users/carlos.araya/code/ruby-book/en/docbook/index.xml:70:43: error: 
element "xi:include" not allowed here; expected the element end-tag or 
element "acknowledgements", "appendix", "article", "bibliography", 
"chapter", "colophon", "dedication", "glossary", "index", "part", 
"preface", "reference" or "toc"
     [java] 
/Users/carlos.araya/code/ruby-book/en/docbook/index.xml:72:52: error: 
element "xi:include" not allowed here; expected the element end-tag or 
element "acknowledgements", "appendix", "article", "bibliography", 
"chapter", "colophon", "dedication", "glossary", "index", "part", 
"preface", "reference" or "toc"
     [java] 
/Users/carlos.araya/code/ruby-book/en/docbook/index.xml:74:40: error: 
element "xi:include" not allowed here; expected the element end-tag or 
element "acknowledgements", "appendix", "article", "bibliography", 
"chapter", "colophon", "dedication", "glossary", "index", "part", 
"preface", "reference" or "toc"
     [java] 
/Users/carlos.araya/code/ruby-book/en/docbook/index.xml:76:39: error: 
element "xi:include" not allowed here; expected the element end-tag or 
element "acknowledgements", "appendix", "article", "bibliography", 
"chapter", "colophon", "dedication", "glossary", "index", "part", 
"preface", "reference" or "toc"
     [java] 
/Users/carlos.araya/code/ruby-book/en/docbook/index.xml:78:34: error: 
element "xi:include" not allowed here; expected the element end-tag or 
element "acknowledgements", "appendix", "article", "bibliography", 
"chapter", "colophon", "dedication", "glossary", "index", "part", 
"preface", "reference" or "toc"

I've checked both the DB5: The Definitive Guide and Bob's book and can't find a reason why this would fail validation. 

Caros