variable using other variable? Works in Saxon, not in Xalan

I have a snippet of code in a stylesheet looking like:

  <xsl:variable name="dbquery">
    <xsl:text>http://127.0.0.1:8080/servlet/db2xml.servlet.DB2XMLServlet?dbQuery=</xsl:text>
    <xsl:text>select%20*</xsl:text>
    <xsl:text>%20from%20collection%20</xsl:text>
    <xsl:text>where%20name='PRICE'%20</xsl:text>
    <xsl:text>and%20dataitemkey%20in%20(</xsl:text>
    <xsl:value-of select="$stocknumbers"/>
    <xsl:text>null)</xsl:text>
  </xsl:variable>

  <xsl:variable name="dbresult" select="document($dbquery)"/>


which constructs an URL in one variable and uses it as an argument to
document() in the next.  I did it with two variables since I otherwise
would need three set of quotes.  $stocknumbers is a string on the form
'1','2','3'

Saxon 5.4.1 executes this as I intended during development from the
command-line.  Xalan 1.2 reports the following error (which I call from 
a servlet which uses processor.process to execute the stylesheet)

XSL Error: pattern = 'document($dbquery)'
VariableReference given for variable out of context or without
definition!  Name = dbquery, source tree node: #document

Which behaviour is correct?  

How can I work around this problem with Xalan?

Thanks in advance for any replies,
-- 
  Thorbj?ørn Ravn Andersen                   "...sound of... Tubular Bells!"
  http://bigfoot.com/~thunderbear




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