xmlSpy gives 'Invalid variable referenceError in XPath

I am trying to transform an docbook-article with a customized stylesheet
and the xmlspy-engine (I am using a university-licence).

I am calling the engine via
AltovaXSLT.exe -xml proposal.xml -xsl docbook_proposal.xsl -out proposal.fo

  Error in D:\docbook_stylesheets\fo\param.xsl at line 255 character 3
  Invalid variable referenceError in XPath expression, Invalid variable
reference
  XSLT stack dump

  xsl:value-of
  xsl:when
  xsl:choose
  xsl:param
  xsl:stylesheet
  xsl:import
  xsl:stylesheet

that would be
       <xsl:value-of select="$page.height.portrait"/>
in param.xsl.

Any help is highly appreciated.
regards
Steffen Gl|ckselig