NativeSelect won't let me unselect a value once set

I have a NativeSelect that won't let me unset a value once it has a value.  I allow null selections and required is false, but while it can be null initially, once a value is chosen, I cannot unselect (have it go back to being a null value).  I tried a value change listener, and only when I select one of the values in the list does it work, but if I try to choose the "blank line" to have no value set again, no value change event occurs.

Basic code to create it looks like this under Vaadin 7.6.5:

if ( propertyId.equals("overridePackageDocumentId") ) {
                    NativeSelect select = new NativeSelect(vaadinUi.getMsg("PackagePartyToDocumentPartyForm.overridePackageDocumentId.label"));
                    select.setDescription(vaadinUi.getMsg("PackagePartyToDocumentPartyForm.overridePackageDocumentId.tooltip"));
                    select.setNullSelectionAllowed(true);
                    select.setImmediate(true);
                    select.setRequired(false);
                    select.setMultiSelect(false);
                    select.addValidator(new SelectValidator(select));
                    select.setItemCaptionMode(ItemCaptionMode.EXPLICIT);

                    Collection<Library> libraries = Library.Manager.getForUserWithViewDetailsPermission(vaadinUi.getUser(), Library.INCLUDE.ONLY_ENABLED);

                    for( Library library : libraries ) {
                        Collection<DocumentInfo> documents = DocumentInfo.Manager.getAll(library,DocumentInfo.INCLUDE.ONLY_ENABLED);

                        for( DocumentInfo docInfo : documents ) {
                            select.addItem(docInfo.getId());
                            select.setItemCaption(docInfo.getId(), library.getPathName().toString() + "/" + docInfo.getEsfName().toString());
                        }
                    }

                    return select;
                }

Answered my own question!

I was missing the select.setNullSelectionItemId(nullId) that specifies the ID to return when nothing is selected.