newbie needs help with xi:include

Hi, 

I want to migratie a couple of my books to docbook 5. I have exported them
to docbook using OpenOffice. But I get an error regarding the xi:include
element. Can someone tell me what I am doing wrong please? 

file1: 
==============================
<book xmlns="http://docbook.org/ns/docbook"
      xmlns:xi="http://www.w3.org/2001/XInclude" version="5.0">
<xi:include href=""/> 
</book>

command issued and error message:
====================================
$ xmlstarlet val --err --xsd
/usr/share/xml/docbook/schema/xsd/5.0/docbook.xsd book.xml 
book.xml:3: element include: Schemas validity error : Element
'{http://www.w3.org/2001/XInclude}include': This element is not expected.
Expected is one of ( {http://docbook.org/ns/docbook}title,
{http://docbook.org/ns/docbook}titleabbrev,
{http://docbook.org/ns/docbook}subtitle,
{http://docbook.org/ns/docbook}info,
{http://docbook.org/ns/docbook}glossary,
{http://docbook.org/ns/docbook}bibliography,
{http://docbook.org/ns/docbook}index, {http://docbook.org/ns/docbook}toc,
{http://docbook.org/ns/docbook}dedication,
{http://docbook.org/ns/docbook}acknowledgements ).
book.xml - invalid


All help greatly appreciated!

kind regards,

Jeroen Baten