biblio collection and HTML transform errors

I'm guessing that I'm missing something simple, but here's the  
problem I'm having...



I'm using a single biblio collection for all of my coursework,  
defined in my customization layer as:



     <!-- Grab my biblio collection -->
     <xsl:param name="bibliography.collection"
         >file:///Users/dnedrow/Documents/xml/biblio.xml</xsl:param>

I have a reference in my text as follows:

Practically Networked<xref linkend="pracnetxp"/>

And the biblio setup in the document is:

<bibliography>
   <bibliomixed id="pracnetxp"/>
</bibliography>

This works fine when transforming to PDF, but when transforming to  
HTML, I get the following error as part of the HTML output:
[pracnetxp] Error: no bibliography entry: pracnetxp found in http:// 
docbook.sourceforge.net/release/bibliography/bibliography.xml



What is it that.... hmmm, am I missing an HTML customization?



-David