Tricky Exception with XML

Hello!

I run this code:
    public String getDocumentAsString(boolean includeInternalDTD) throws IOException, LibraryException
    {
        StringWriter sw = new StringWriter();
        ((TXDocument)_document).setPrintInternalDTD(includeInternalDTD);
        ((TXDocument)_document).printWithFormat(sw);

        return(sw.toString());
    }

I call it with: compiler.getDocumentAsString(true); and
compiler.getDocumentAsString(false);

And I allways get this error:
com.ibm.xml.parser.LibraryException: com.ibm.xml.parser.TXDocument#printWithForm
at(): Unexpected Exception: java.lang.NullPointerException
        at com.ibm.xml.parser.TXDocument.printWithFormat(TXDocument.java)
        at com.ibm.xml.parser.TXDocument.printWithFormat(TXDocument.java)
        at com.my.test.xml.XMLCompiler.getDocumentAsString(XMLCompil
er.java:294)

Does any one understand why?

please help