Binding an OptionGroup to List

Hello Guys,
We use an OptionGroup to let a User select one or more Items. The OptionGroup has a Binder to a HashSet. Now every time we try to commit, we get the following Exceptions:
com.vaadin.data.fieldgroup.FieldGroup$CommitException: Commit failed
com.vaadin.data.util.converter.Converter$ConversionException: Could not convert value to Set

We already tried to write a Converter for it, but this doesnt work either:

[code]
public class RollenConverter implements Converter<Object, Set> {

 private BeanContainer<Integer, Rolle> container;
 private final static Set<Rolle> MODEL_TYPE_INSTANCE = new HashSet<Rolle>();
 private OptionGroup opg;

 public RollenConverter(AbstractSelect field) {
    this.container = (BeanContainer<Integer, Rolle>) field.getContainerDataSource();
    opg=(OptionGroup) field;
 }

 @Override
 public Set<Rolle> convertToModel(Object value, Class<? extends Set<Rolle>> targetType, Locale locale)
        throws com.vaadin.data.util.converter.Converter.ConversionException {
    
    Set<Rolle> models = new HashSet<Rolle>();

    if (container.size() > 0) {
        for (Object itemId : ((HashSet<Object>) value)) {
            if (container.containsId(itemId)) {
                models.add(container.getItem(itemId).getBean());
            }
        }
    }
    return models;
    
 }

 @Override
 public Object convertToPresentation(Set<Rolle> value, Class<? extends Object> targetType, Locale locale)
        throws com.vaadin.data.util.converter.Converter.ConversionException {
    
    for(Rolle r:value){
        if(opg.containsId(r.getId())){
            opg.select(r.getId());
        }
    }
    
    return value;
}

@Override
public Class<Set<Rolle>> getModelType() {
     return (Class<Set<Rolle>>) MODEL_TYPE_INSTANCE.getClass();
}

@Override
public Class<Object> getPresentationType() {
    return Object.class;
}

}
[/code]Has someone an idea whats wrong? Please help