[ERROR] org.apache.fop.apps.FOPException: fo:flow must contain block-level child

I´m trying to make a modular book, using ENTITY to import all book
segments, but the FOP produces the following error:
[ERROR] org.apache.fop.apps.FOPException: fo:flow must contain block-level child

bellow comes my test documents:

BOOK =============================================
<?xml version="1.0" encoding="ISO-8859-1"?>
<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
               "http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" [
<!ENTITY % bug SYSTEM "bug.xml">
]>
<book id="book">
<preface id="forward"><title>Foreword</title>
<para>A preface.</para>
</preface>
<part id="gettingStarted">
	<title>Preparing the writing environment</title>
	&bug;
</part>
</book>
+++++++++++++++++++++++++++++++++++++++++++++++++++

CHAPTER ===========================================
<?xml version="1.0" encoding="ISO-8859-1"?>
<!DOCTYPE chapter PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
               "http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd">
<chapter id="chapter">
	<title></title>
	<titleabbrev>Installing the XSLT processors</titleabbrev>
	<section id="xsltproc">
		<title>xsltproc</title>
		<titleabbrev>xsltproc</titleabbrev>
		some tips about the project tree
	</section>
</chapter>
++++++++++++++++++++++++++++++++++++++++++++++++++++

several hours trying variations of the above examples....

any tip ?

   Felipe Gaúcho