Is it illegal to use a Variable name in a key?

I need help on using xsl:key
I have a strange behaviour with XSL:KEY. 
The following key creates a problem with Websphere 4.0.3 but works fine in 
Weblogic and Tomcat.

<xsl:variable name="asRevRevRelation" select="'AsRevRev'"/>
<xsl:key name="asRevRevRelations"
         match="/ObjectSetRoot/Object[(_at_)Class=$asRevRevRelation]"
         use="child::DBE:Attribute[(_at_)name='Left']/*"/>

ON removing the $asRevRevRelation variable reference, the key works fine.



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