xsl:when conditions

Hello,
  In the XSL below I'm trying to pass a optional parameter called "alignment" 
into a template. When the parameter is not passed into the template I want to 
assign a default value (left) to the variable "alignment"

  The line <xsl:when test="$alignment"> seems to be causing the problem and 
did work under Xalan 1_2_2, however it fails under Xalan 2_2 with "Unknown 
error in XPath"

  Could someone explain how best to set a default value to the parameter ?

<?xml version="1.0"?>
<xsl:stylesheet    xmlns:xsl="http://www.w3.org/1999/XSL/Transform" 
version="1.0"    xmlns:fo="http://www.w3.org/1999/XSL/Format">
<xsl:template match ="/">

<!-- Text aligned right -->
 <xsl:call-template name="writeline">
	<xsl:with-param name="text" select="string('some text')"/>
	<xsl:with-param name="alignment" select="string('right')"/>
 </xsl:call-template>

<!-- Should take default -->
 <xsl:call-template name="writeline">
 	<xsl:with-param name="text" select="string('some more text')"/>
 </xsl:call-template>

</xsl:template>

<xsl:template name="writeline">
<xsl:param name="text"/>
<xsl:param name="alignment"/>
<xsl:variable name="alignment">
	<xsl:choose>

		<xsl:when test="$alignment"> <===== This syntax causes error
 			<xsl:value-of select="$alignment"/>
		</xsl:when>
 		<xsl:otherwise>
 			<xsl:value-of select="string('left')"/>
 		</xsl:otherwise>
 	</xsl:choose>
 </xsl:variable>


<table width="100%" border="1">
<tr>
<td align="{$alignment}"><xsl:value-of select="$text"/></td>
</tr>
</table>
</xsl:template>

</xsl:stylesheet>


Regards,

Stuart

-----------------------
The Totalise Email system, probably the most flexible email system in the
world. To register for an account goto http://www.totalise.net


 XSL-List info and archive:  http://www.mulberrytech.com/xsl/xsl-list