Problem about secondly run "gtkdoc-mkdb"

I'm using gtk-doc(1.11) to manager documents of my small project. I encounter some problems like: "../xml/foo.xml:161: parser error : Entity 'ast' not defined", when secondly running gtk-doc.

The steps are:

1) gtkdoc-scan --module=foo --source-dir=../../foo --ignore-headers="a.h b.h"

2) gtkdoc-mkdb --module=foo --source-dir=../../foo --output-format=xml --expand-content-files="" --main-sgml-file=foo-docs.sgml --sgml-mode --output-format=xml --ignore-files="a.c b.h"

...

I find in the second time, gtkdoc-mkdb generates XML files missing the following:

<?xml version="1.0"?>

<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.3//EN"

               "http://www.oasis-open.org/docbook/xml/4.3/docbookx.dtd">

Any suggestions?

Thanks.