Error in Docbook XSL stylesheets?

["Paul Suckling" <paul.suckling@gmail.com>]

Dear all.

I was turning a fairly large DocBook XML document into PDF just fine
for some time using DocBook XML and Apache FOP. I am currently using
the latest DocBook XSL files (1.74.0) and Apache FOP 0.94.

Then, after some changes to the document (I haven't been able to
identify exactly what yet), I tried to create a new PDF and got the
following error.

runtime error: file /usr/share/docbook-xsl-latest/fo/spaces.xsl line
267 element value-of
Internal error in xsltValueOf(): failed to cast an XPath object to string.

Line 267 is the line I have marked with a comment in the following
template block:

<xsl:template name="space.200A.subst">
  <xsl:param name="string"/>

  <xsl:choose>
    <xsl:when test="contains($string, '&#x200A;') and
$space.hairspace.width != ''">
      <xsl:value-of select="substring-before($string, '&#x200A;')"/>
      <fo:leader leader-length="{$space.hairspace.width}"/>
      <xsl:call-template name="space.200A.subst">
	<xsl:with-param name="string" select="substring-after($string, '&#x200A;')"/>
      </xsl:call-template>
    </xsl:when>
    <xsl:otherwise>
      <xsl:value-of select="$string"/> <!-- LINE 267 -->
    </xsl:otherwise>
  </xsl:choose>
</xsl:template>


I haven't yet been able to figure out what the problem is. I don't
know if there is an error in the document or if his is an error in the
XSL stylesheets. I don't know enough about the stylesheets and XSLT to
know what this block is supposed to be doing.

Can someone help me out here please?

Thank you,

Paul