XInclude mess

I惴 trying to configure a modular documentation ... but something is wrong:

the INCLUDE command I惴 using:

<xi:include xmlns:xi="http://www.w3.org/2001/XInclude"  href="chapter.xml" />

The CHAPTER code:

<?xml version="1.0" encoding="ISO-8859-1"?>
<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
               "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
<chapter id="chapter">
	<title>Testing the environment</title>test
</chapter>

the ERROR:
[INFO] [7]
file:///C:/docbook-xsl-1.66.1/fo/docbook.xsl; Line #89; Column #16; xi:include e
ncountered in part, but no template matches.
[ERROR] java.lang.NullPointerException

any tip?