How to make preface not a chunk?

[Boris <boriss@web.de>]

I've a huge customization layer turning a DocBook 5 documentation into a  
couple of HTML files. I'd like to change the stylesheets now to make the  
preface not a chunk. I found the template "chunk" in chunk-common.xsl and  
changed line 1002 to:

<xsl:when test="local-name($node)='preface'">0</xsl:when>

When I process my XML files then I get this error message though:

Error preface is not a chunk!
Writing ../bla/ for preface
I/O error : Is a directory
I/O error : Is a directory
runtime error: file /home/Boris/docbook-xsl-ns-1.74.0/html/chunker.xsl  
line 136 element document
xsltDocumentElem: unable to save to ../bla/

Does anyone know where the error message comes from (I searched for "is  
not a chunk" but couldn't find anything in the XSL files)? Or - even  
better - does anyone know how to make the preface not a chunk?

Boris