Fix Validation fails when using XML (ClassCastException)

IOIValidation.java

package fixfun;

import quickfix.DataDictionary;
import quickfix.DefaultDataDictionaryProvider;
import quickfix.FieldNotFound;
import quickfix.IncorrectDataFormat;
import quickfix.IncorrectTagValue;
import quickfix.field.ApplVerID;
import quickfix.field.Currency;
import quickfix.field.IOIQty;
import quickfix.field.IOITransType;
import quickfix.field.IOIid;
import quickfix.field.SecurityID;
import quickfix.field.SecurityIDSource;
import quickfix.field.SenderCompID;
import quickfix.field.Side;
import quickfix.field.Symbol;
import quickfix.field.TargetCompID;
import quickfix.field.XmlData;
import quickfix.field.XmlDataLen;
import quickfix.fix50.IOI;

public class IOIValidation {

    private void validateIoiWithoutXml() {
        final IOI ioi = createAValidIoi();
        validateIoi(ioi);
    }

    private void validateIoiWithXml() {
        final IOI ioi = createAValidIoi();

        final String xml = "<a>Hello</a>";
        ioi.getHeader().setField(XmlData.FIELD, new XmlData(xml));
        ioi.getHeader().setField(XmlDataLen.FIELD, new XmlDataLen(xml.length()));

        validateIoi(ioi);
    }

    private void validateIoi(final IOI ioi) {
        System.out.println(ioi.toString());
        final DefaultDataDictionaryProvider d = new DefaultDataDictionaryProvider();
        final DataDictionary dict = d.getApplicationDataDictionary(new ApplVerID(ApplVerID.FIX50));
        try {
            dict.validate(ioi);
        } catch (final IncorrectTagValue e) {
            e.printStackTrace();
        } catch (final FieldNotFound e) {
            e.printStackTrace();
        } catch (final IncorrectDataFormat e) {
            e.printStackTrace();
        }
    }

    private IOI createAValidIoi() {
        final IOI ioi = new IOI();
        ioi.getHeader().setField(SenderCompID.FIELD, new SenderCompID("Sender"));
        ioi.getHeader().setField(TargetCompID.FIELD, new TargetCompID("Target"));
        ioi.set(new IOIid("IOD-1"));
        ioi.set(new IOITransType(IOITransType.NEW));
        ioi.set(new Side(Side.SELL));
        ioi.set(new IOIQty(IOIQty.MEDIUM));
        ioi.set(new Currency("USD"));

        ioi.set(new Symbol("IBM.N"));
        ioi.set(new SecurityIDSource(SecurityIDSource.RIC_CODE));
        ioi.set(new SecurityID("IBM.N"));
        return ioi;
    }

    public static void main(final String[] args) {
        final IOIValidation test = new IOIValidation();

        System.out.println("Testing WITHOUT XML");
        test.validateIoiWithoutXml();

        System.out.println("Testing WITH XML ===> FAILS");
        test.validateIoiWithXml();
    }
}