Error resolving the default value of a parameter in x

I am getting an error as I try to pass the xsl:sort order of a list  via a
URL parameter in Cocoon.  The style sheet receives two parameters, and sets
default values like this:
...
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
version="2.0">
   <xsl:param name="sortBy" select="id"/>
   <xsl:param name="sortOrder" select="ascending" />
...

Later on, I sort a list using:
<xsl:sort select="(*|*/*)[name()=$sortBy]" order="{$sortOrder}"/>

Everything works fine if $sortOrder has been passed through the URL (i.e.,
http://example/home?sortBy=name&sortOrder=descending). However, if
$sortOrder is not set in the URL, I get an error, "order must be 'ascending'
or 'descending'."  My understanding is that it should use the default value
of 'ascending', and indeed it appears to correctly resolve the default value
of $sortBy when needed.

Thanks, Andy