DOCBOOK: imagedata and entityref

This

  <!DOCTYPE article PUBLIC  '-//OASIS//DTD DocBook V3.1//EN' [
  <!ENTITY preprocessor SYSTEM "preprocessor.gif">
  ]>
  <article>
    <mediaobject>
      <imageobject>
	<imagedata entityref="preprocessor">
      </imageobject>
    </mediaobject>
  </article>

gives me this error

  jade -t sgml -d /usr/share/sgml/docbook/dsssl-stylesheets/html/docbook.dsl test.sgml
  jade:test.sgml:7:28:E: "preprocessor" is not a data or subdocument entity

I am using

  docbook-dtd31-sgml-1.0-10
  openjade-1.3-13
  docbook-utils-0.6-13
  docbook-style-dsssl-1.64-2
  jadetex-3.3-1

on Red Hat 7.1 Linux.

I'm quite a bit fuzzy about entities and how they work, so any help
would be appreciated.

Thanks.

-- Vladimir