Sort problem

Hi,
I've a problem during sort node with xalan-j version 2.4.1(bundle with jdk 
1.4.2_05). 
I have an XML like this:
=======================
<?xml version="1.0"?>
<Peoples>
    <Person>
        <Name>Johnson</Name>
        <Level>foo</Level>
    </Person>
    <Person>
        <Name>Smith</Name>
        <Level>baz</Level>
    </Person>
    <Person>
        <Name>White</Name>
        <Level>foo</Level>
    </Person>
    <Person>
        <Name>Brown</Name>
    </Person>
    <Person>
        <Name>Black</Name>
        <Level>bar</Level>
    </Person>
</Peoples>
=======================
and I want sort by level and then by name giving a rule for mapping level with 
an arbitrary number.
I wrote an xml like this:
=======================
<?xml version="1.0"?>
<xsl:stylesheet  xmlns:xsl="http://www.w3.org/1999/XSL/Transform"; version="1.0"
  xmlns:this="http://localhost/"; exclude-result-prefixes="this"
>
 
<this:Levels>
    <Level pos="0"/><!--dummy level-->
    <Level pos="1">foo</Level>
    <Level pos="2">bar</Level>
    <Level pos="3">baz</Level>
</this:Levels>
 
<xsl:variable name="levels" select="document('')/xsl:stylesheet/this:Levels"/>
 
<xsl:template match="/Peoples">
    <xsl:apply-templates select="Person">
        <xsl:sort data-type="number" 
select="number($levels/Level[.=current()/Level]/@pos)"/>
        <xsl:sort select="Name"/>
    </xsl:apply-templates>
</xsl:template>
 
<xsl:template match="Person">
    <xsl:value-of select="Name"/><br/>
</xsl:template>
 
</xsl:stylesheet>
=======================
 
during transfom, the element  <xsl:sort data-type="number" 
select="number($levels/Level[.=current()/Level]/@pos)"/> produce an excepiton 
javax.xml.transform.TransformerException: java.lang.NullPointerException

but if i print the <xsl:value-of 
select="number($levels/Level[.=current()/Level]/@pos)"/> in "Person" template I 
got the correct value.
If apply the same transform directly with my IE6.0 (so using msxml transformer) 
i obtain the correct result.
 
What's wrong?
It is my error suppose that the current() function inside sort element refer to 
selected node in apply-templates?
 
regards,
Paolo