[docbook] <book> won't accept "xmlns:xi" attribute

I cannot get "include" to work in my XML document. Here's my snippet:

<?xml version='1.0'?>
<!DOCTYPE book SYSTEM "file:///O:/prog/docbook-4.5/dtd/docbook.dtd">
<book xmlns="http://docbook.org/ns/docbook"
         xmlns:xi="http://www.w3.org/2001/XInclude">
<xi:include href="chapter1.xml"/>
</book>

Element include in namespace 'http://www.w3.org/2001/XInclude' encountered
in book, but no template matches.
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