Evaluating parameter in <xsl:when> vs evaluating param

Hi there

In the following snippet the " xalan:evaluate($sortParameter) " gets the
correct value when placed in the  <xsl:sort> tag but not does not
seem to do so when placed in the <xsl:when> tag.
Is there any reason why the evaluation in the <xsl:when> tag is not
working in my code?

....
<xsl:variable name="sortParameter">
      <xsl:value-of select="sortby"/>
</xsl:variable>

 <xsl:choose>
        <xsl:when test="xalan:evaluate($sortParameter) = 'name'" >
              <xsl:for-each select="query">
              <xsl:sort select="xalan:evaluate($sortParameter)"
order="ascending"/>
              ......
        </xsl:when>
<xsl:otherwise>
          <xsl:for-each select="query">
          <xsl:sort select="xalan:evaluate($sortParameter)"
order="descending"/>
....


Thanks in advance
Roelf



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