Parameter evaluation after use of document()

Hi,

I am wondering why the XSLT code below doesn't work. Xalan-J dies with
the (not very helpful) errors mentioned below as soon as it tries to
get the value of "filename_base" in the second last line in the
stylesheet excerpt.

It seems that the parameter value has somehow been lost at that point
- I suspect that the problem is related to the fact that I'm
processing nodes obtained by using document().

FAQs and literature didn't tell much about that topic.

Can anybody give me a clue on what is wrong with my stylesheet? (I've
simplified it a little bit.)

  Regards,

    Yves Forkl

++++++++++ content of file "Chapter_1.xml" ++++++++++

<?xml version="1.0" encoding="iso-8859-1"?>
<!-- ... -->
<!-- ... -->
<chapter id="Chapter1_ID">
<title>My First Chapter</title>
...
</chapter>

++++++++++ excerpt of stylesheet ++++++++++

<!-- value of $type is not relevant -->

<!-- process all element nodes of type $type -->
<xsl:for-each select="key('select_nodes_by_type', $type)">
  <xsl:call-template name="generic_info_template">
    <!-- extract the text node of the each element node of type $type
-->
    <xsl:with-param name="filename_base" select="text()"/>
    <!-- SUPPOSE: 1st run: $filename_base = "Chapter_1" -->
  </xsl:call-template>
</xsl:for-each>

<!-- apply appropriate info_mode templates to file contents -->
<xsl:template name="generic_info_template">
  <xsl:param name="filename_base"/>
  <!-- read in file contents and process them in info_mode -->
  <xsl:apply-templates
    select="document(concat($filename_base, '.xml'))"
    mode="info_mode">
    <!-- hand the param to the called template -->
    <xsl:with-param name="filename_base"
      select="$filename_base"/>
  </xsl:apply-templates>
</xsl:template>

<!-- write out info specifically for chapter nodes -->
<xsl:template match="chapter" mode="info_mode">
  <xsl:param name="filename_base"/>
  <xsl:text>This chapter's title: </xsl:text>
  <xsl:value-of select="title"/>
  <xsl:text>
This chapter's filename base: </xsl:text>
  <xsl:value-of select="$filename_base"/>
</xsl:template>

++++++++++ fatal error messages by Xalan-J ++++++++++

(always occurring at the above stylesheet's second last line)

* with option -TS:

  (default root rule) apply-templates, select='null':
       1000001: #comment
       1000002: #comment
       1000003: chapter
  
  (Location of error unknown)XSLT Error
  (javax.xml.transform.TransformerException)
  : java.util.EmptyStackException

* with options -TG or -TT or -TTC:

  (Location of error unknown)XSLT Error
  (javax.xml.transform.TransformerException)
  : java.lang.reflect.InvocationTargetException

 XSL-List info and archive:  http://www.mulberrytech.com/xsl/xsl-list