cannot assign val to sort key

>From an asp page I pass values for sort key and sort
order into variables in my xsl:

  <xsl:variable name="sortKey"   />
  <xsl:variable name="sortOrder" />

I know this part works b/c I've printed out their 
values. But I can't seem to assign assign the values
of these variables to my <xsl:sort>. Here's what I'm
doing:

<xsl:sort select="$sortKey" order="{$sortOrder}" />

$sortKey is the variable name, but it doesn't get 
picked up... what am I diong wrong?

thanks in advance,
-alex

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