[xsl] 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[@Class=$asRevRevRelation]"
         use="child::DBE:Attribute[@name='Left']/*"/>

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



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