xsl:key use attribute using string() causes missing al

Hi,

I recently had an interesting discovery with xsl:key
use attribute using string().
I been using string() in xsl:key use attribute
extensively since it allows me to retrieve records
using the key value "".  But in the following example,
using string() caused me to miss all matches except
the first one.  I am not sure what is the reason for
this, would appreciate if anyone could shed light on
this behavior.  Example input XML and XSLT below.

thanks,
Xiaocun
	
input XML:
<range>
	<row row="16">
		<cell column="1">supplier</cell>
		<cell column="2">s3</cell>
		<cell column="3">item</cell>
		<cell column="4">
			<subcell>item1</subcell>
			<subcell>item2</subcell>
			<subcell>item3</subcell>
		</cell>
		<cell column="5">s3_AM1</cell>
		<cell column="6">accepted</cell>
	</row>
	<row row="17">
		<cell column="1">supplier</cell>
		<cell column="2">s4</cell>
		<cell column="3">item</cell>
		<cell column="4">
			<subcell>item1</subcell>
			<subcell>item2</subcell>
			<subcell>item4</subcell>
		</cell>
		<cell column="5">s4_AM1</cell>
		<cell column="6">accepted</cell>
	</row>
	<row row="18">
		<cell column="1">supplier</cell>
		<cell column="2">s5</cell>
		<cell column="3">item</cell>
		<cell column="4">item1</cell>
		<cell column="5">s5_AM1</cell>
		<cell column="6">accepted</cell>
	</row>
</range>

complete XSLT without using string():
<xsl:stylesheet version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
xmlns:fo="http://www.w3.org/1999/XSL/Format">
<xsl:key name="itemInvitesKey"
match="//row[cell[@column=3] = 'item']"
use="cell[@column=4]"/>
<xsl:key name="itemInvitesKey"
match="//row[cell[@column=3] = 'item']"
use="(cell[@column=4]/subcell)"/>
<xsl:template match="/">
	<xsl:apply-templates select="range"/>
</xsl:template>
<xsl:template match="range">
	<xsl:text>item1&#xA;</xsl:text>
	<xsl:for-each select="key('itemInvitesKey',
'item1')">
		<xsl:value-of select="cell[@column=2]"/>
		<xsl:text>&#xA;</xsl:text>
	</xsl:for-each>
	<xsl:text>&#xA;</xsl:text>
	<xsl:text>item2&#xA;</xsl:text>
	<xsl:for-each select="key('itemInvitesKey',
'item2')">
		<xsl:value-of select="cell[@column=2]"/>
		<xsl:text>&#xA;</xsl:text>
	</xsl:for-each>
</xsl:template>
</xsl:stylesheet>

correct output:
item1
s5
s3
s4

item2
s3
s4

Exact same XSLT as above, except xsl:key use attribute
using string():
<xsl:key name="itemInvitesKey"
match="//row[cell[@column=3] = 'item']"
use="string(cell[@column=4]/subcell)"/>

Output: same as above, except no matches were returned
except for the first key (item1)
item1
s5
s3
s4

item2

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