Standalone XML with parser from IBM

Hello!
(All XML-classes used in this can be found at)
http://www-3.ibm.com/software/webservers/appserv/doc/v35/ae/apidocs/index.html

I receive a XML-structure thats look like:
<?xml version="1.0" standalone="yes"?>
<product>
    <header>
        <message_id_send>511114351</message_id_send>
        <message_id_get>511114351</message_id_get>
<error_message></error_message>...

I try to read it with:
StringReader sr = new StringReader(XMLString);
Parser parser = new Parser("test");
Document document = parser.readStream(sr);

Then i try to get a element:
getFieldValue("error_number", _document);

Ande the method getFieldValue looks like:
    protected String getFieldValue(String tagname, Document document)
    {
        String value = null;

        try
        {
            NodeList nodeList = document.getElementsByTagName(tagname);
            Element node = (Element) nodeList.item(0);
            try
            {
                value = node.getFirstChild().getNodeValue();
            }
            catch(NullPointerException npe)
            {
                value = null;
            }
        }
        catch(Exception e)
        {
            e.printStackTrace(System.out);
        }
        return value;
    }
But I always get this message at line with Element node = (Element) nodeList.item(0);:

file:/D:/Inetpub/fap_se/Java/test: 1, 108: Element, "message_id_send" is not dec
lared in the DTD
java.lang.ArrayIndexOutOfBoundsException: 0 >= 0
        at java.util.Vector.elementAt(Vector.java:427)
        at com.ibm.xml.parser.TXNodeList$VectorNodeList.item(TXNodeList.java)
        at com.fap.xml.DataStorer.getFieldValue(DataStorer.java:94)
        at com.fap.xml.If.IfDataStorer.isDocumentOk(IfDataStorer.java:767)
        at com.fap.xml.If.IfDataStorer.store(IfDataStorer.java:80)
        at com.fap.xml.XMLManager.transmit(XMLManager.java:517)
        at com.fap.xml.XMLManager.run(XMLManager.java:152)
        at java.lang.Thread.run(Thread.java:536)

So if some one know how to tell the parser to parse without DTD as the xml-head says - standalone, please let me know.

Best Regards.

PS I can not use an other Parser since this is in a big project with a lot of developers.