Xalan giving TransformerException for a defined variab

I'm working with ATG Dynamo application server, attempting
to upgrade from a version that used a very old, pre- 1.0 certified
XSL parser, "LotusXSL Version 0.18.4, XML4J Version XML4J 2.0.15 ",
to a version that uses Xalan, "Xalan Version Xalan Java 2.4.1".

A stylesheet that worked with the Lotus parser blows up
when processed by Xalan, a NullPointerException when dealing
with a param variable. Has anyone seen this problem before,
or is this in some way illegal XSL the old parser allowed?

i'm attaching the stylesheet reduced down to the basics
that still blows up. if one removes the "$shippedOrder"
variable in the <xsl:sort> element, then the Xalan processor
will not blow up. also attaching the stacktrace.
Thanks for any help

------------- The XSL stylesheet -----------------
<?xml version="1.0"?>
<xsl:stylesheet version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform" >

<xsl:template match="ORDER_STATUS_DOCUMENT">
<xsl:element name="Shipments">
<xsl:apply-templates select="ORDER[ORDER_DETAIL/ITEM_STATUS/SHIP]"/>
</xsl:element>
</xsl:template>

<xsl:template match="ORDER">
<xsl:element name="Shipment">
<xsl:call-template name="generate_containers">
<xsl:with-param name="shippedOrder" select="."/>
</xsl:call-template>
</xsl:element>
</xsl:template>

<!-- key to generate a node-set of shipment elements that have a common
tracking number, hence should be in the same Container -->
<xsl:key name="container-group" match="//PACKAGE" use="TRACKING_NUMBER"/>

</xsl:stylesheet>
------------- END of The XSL stylesheet -----------------