X-Path function not working

Hi, I use MII15 and got a document having the following structure:

<?xml version="1.0" encoding="UTF-8"?>

...

    <Rowset>

        <Row>

            <WERKS>ZZ01</WERKS>

            <ORDER>001200306732</ORDER>

            <WORK_CEN_LN_SEG>ST_04_01</WORK_CEN_LN_SEG>

            <ROUTING_SEQ_LINE>000000</ROUTING_SEQ_LINE>

            <OPER_ACT_PT>0001</OPER_ACT_PT>

            <CONF_QTY>0</CONF_QTY>

            <OPER_QTY>200</OPER_QTY>

            <REP_DATE>0</REP_DATE>

            <REP_TIME>0</REP_TIME>

            <NEXT_PLNF>000000</NEXT_PLNF>

            <NEXT_OPER>0005</NEXT_OPER>

        </Row>

        <Row>

...

I would like to only get the Row node having WORK_CEN_LN_SEG ST_04_01.

To check if my appraoch is working, I mapped XPath Output_Data.Output{/Rowsets/Rowset/Row[WORK_CEN_LN_SEG = 'ST_04_01']} to the message of a Tracer action block...But it remains emtpy:

[INFO] [Tracer_1]

As far as I understood, it should return above node as that work center is key in that document. Any idea what I am doing wrong?

Thanks for help, Marco