Unselectable null value in NativeSelect

Creating a new NativeSelect component with null selection allowed lists the empty value correctly, but it cannot be selected. Clicking the empty value doesn't even fire a new ValueChangeEvent. However, setting an item ID for null value with setNullSelectionItemId(<id>) makes the null value selectable. Is this behaviour as intended or am I doing something wrong?
Tested with the following code on a new Vaadin 7.6.4 project:

NativeSelect ns = new NativeSelect("Native select");
        ns.addItem("item1");
        ns.addItem("item2");
        ns.setValue("item1");
        ns.setNullSelectionAllowed(true);
        ns.setImmediate(true);
        layout.addComponent(ns);