Xmllint error

Hi,

When compiling my modular docbook file I get this xmllint error message.
What am I doing wrong?

$ xmllint --xinclude --postvalid IDS5webUGbook.xml > out.xml
IDS5web/1_Introduction/chapter_all_introduction.xml:3: No declaration for
attribute base of element chapter .
.
.
Document 3D_user_guide_book.xml does not validate

My book-file:
<?xml version="1.0"?>
<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
"C:/tools/lib/docbook/xml/42/docbookx.dtd">

<!--http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd-->

<book>

<bookinfo>
stuff
</bookinfo>

<xi:include xmlns:xi="http://www.w3.org/2001/XInclude"
href="IDS5web/1_Introduction/chapter_all_introduction.xml" />

More modules ....

</book>

A module:
<?xml version="1.0" encoding="UTF-8"?>
<chapter>
<title>Introduction </title>
<xi:include xmlns:xi="http://www.w3.org/2001/XInclude"
href="document_conventions.xml" /> <xi:include
xmlns:xi="http://www.w3.org/2001/XInclude" href="precautions.xml" />
<xi:include xmlns:xi="http://www.w3.org/2001/XInclude"
href="system_usage_safety_regulations.xml" /> <xi:include
xmlns:xi="http://www.w3.org/2001/XInclude" href="using_help.xml" />

</chapter>

Thanks,

/Kenneth

--------------------------------------------------------
Kenneth Johansson	Technical Documentation Manager
Sectra Imtec AB
Teknikringen 2		E-mail: ke-joh AT sectra DOT se
SE-583 30 Link鞿ing	Phone:	+46 13 23 52 00
SWEDEN			Web:	http://www.sectra.se