What's wrong with this key?

I'm currently using keys with Xalan to group course-data by week of the year.

<xsl:key name="WosNachWoche" match="//KURS[VERANART='Wochenendseminar' and not(AUSFALL)]"
        use="datetime:weekInYear(BEGINNDAT))" />

Now I need to enhance grouping to work with courses from different
years, so I figured I'd just need to extend the use-term like this:

<xsl:key name="WosNachWoche" match="//KURS[VERANART='Wochenendseminar' and not(AUSFALL)]"
        use="concat(datetime:weekInYear(BEGINNDAT),':',datetime:formatDate(BEGINNDAT,'y'))" />

Unfortunately this isn't working for some reason I don't understand.
For quick testing I use this template


<xsl:template match="/">
    <xsl:for-each select="key('WosNachWoche',//*)">
        <xsl:message>
            <xsl:value-of select="TITELKURZ"/>
        </xsl:message>
    </xsl:for-each>
</xsl:template>

and found that the second use-term gives me NO results. Exploring
further it seems like as soon as I use concat in the use-expression I
stop getting any results.

Any ideas? What is the problem and how can I assemble a key with
weeknumber and year?

Thanks for any help,

Ferdinand Soethe