use variable as node-set for key get NullPointerExcept

Hi,

	I declared a variable and then use the variable as
the node-set for key, I get a NullPointerException
with Saxon:
	
<!-- Find all lineitems, key on LineItemLotName -->
<xsl:variable name="LineItems">
	<xsl:choose>
		<!-- if there are bids under the line items, process
up to bid header -->
		<xsl:when test="$BidHeaderRow &gt; 0">
			<xsl:apply-templates
select="following-sibling::row[@row &gt;
$LineItemHeaderRow and @row &lt; $BidHeaderRow]"
mode="LineItem"/>
		</xsl:when>
		<xsl:otherwise>
			<xsl:apply-templates
select="following-sibling::row[@row &gt;
$LineItemHeaderRow]" mode="LineItem"/>
		</xsl:otherwise>
	</xsl:choose>
</xsl:variable>
<xsl:variable name="LineItemLotColumn" select="6"/>
<xsl:key name="lineitemLotsKey"
match="$LineItem[cell[@column=$LineItemLotColumn]]"
use="cell[@column=$LineItemLotColumn]"/>

I get the following exception:
java.lang.NullPointerException
        at
com.icl.saxon.KeyManager.constructIndex(KeyManager.java:105)
        at
com.icl.saxon.KeyManager.buildIndex(KeyManager.java:83)
        at
com.icl.saxon.KeyManager.selectByKey(KeyManager.java:185)
        at
com.icl.saxon.functions.Key.findKey(Key.java:100)
        at
com.icl.saxon.functions.Key.enumerate(Key.java:68)
        at
com.icl.saxon.functions.Key.evaluate(Key.java:50)
        at
com.icl.saxon.expr.Expression.enumerate(Expression.java:156)
        
How to best work around this problem?

Thanks,

Xiaocun
xiaocunxu@xxxxxxxxx

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