my stylesheet
-------------
<?xml version="1.0"?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output method = "html" encoding="iso-8859-1" />

<xsl:variable name = "x" select = "'1'" />
<xsl:variable name = "y" select = "'4'" />
<xsl:key name = "key_test" match ="test" use = "substring(date, $x, $y)" />

<xsl:template match="/">

</xsl:template>
</xsl:stylesheet>

xml file
<test><date>2004-01-01</date></test>


java.lang.NullPointerException
at java.lang.FloatingDecimal.readJavaFormatString(FloatingDecimal.java:980)
at java.lang.Double.valueOf(Double.java:202)
at org.apache.xalan.xsltc.runtime.BasisLibrary.stringToReal(BasisLibrary.java:838)
at ent_recherche_simple.buildKeys()
at ent_recherche_simple.transform()
at org.apache.xalan.xsltc.runtime.AbstractTranslet.transform(AbstractTranslet.java:580)