Adding a new element to docbook

I'm still working on adding a new element to docbook, and am becoming
pretty confused. Here's what I've got so far:

This is mydocbk.dtd:

<!ENTITY % local.tech.char.class "footer">
<!ENTITY % DocBookDTD PUBLIC "-//OASIS//DTD DocBook V4.2//EN">
%DocBookDTD;

<!ELEMENT footer - - ((%para.char.mix;|%para.mix;)+)>


This is my test xml file:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE book SYSTEM "file:///C:/DocBook/XSLTest/DTD/mydocbk.dtd">
<book><title>Book Title</title>

<chapter><title>Using the footer element</title>
<para>This is only a sample.</para>
<footer>This is the footer</footer>
</chapter>
</book>


I am receiving an error that states "Missing white space before SYSTEM
literal URI."

Thanks in advance for any assistance.

-JB