Mapping via intermediate XML file problem

Hi.

I'm doing a simple transformation, substituting some tags with
different named ones and keeping the rest the same.
Since there are many strange cases, I'd like to keep the mapping in
the intermediate XML file (to keep the XSL clean), such as maps.xml:

<?xml version="1.0" encoding="UTF-8"?>
<maps>
    <map old="old-tag1" new="tag1"></map>
    <map old="old-strange-tag2" new="tag2"></map>
    <!-- <map old="" new=""></map> -->
</maps>

and then transform using:

<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="2.0">

<xsl:variable name="maps" select="document('maps.xml')/maps/map"></xsl:variable>

<xsl:template match="*">
    <xsl:element name="{$maps[@old = local-name(current())]/@new}">
        <xsl:apply-templates select="node()|@*"/>
    </xsl:element>
</xsl:template>

</xsl:stylesheet>

But when I run that through Saxon 9B it says "Invalid element name.
Invalid QName {}" URL: http://www.w3.org/TR/xslt20/#err-XTDE0820

My source file contains both the elements matched by map and other elements too.

Any pointers deeply appreciated,

Lech