Icon recources set via combobox.itemIconGenerator not loaded - why?

Vaadin 6 / 7 / 8
1

What could be causing icon recources set via combobox.itemIconGenerator not to be loaded?

                comboBox.setItemIconGenerator((aLanguageChoice) -> {
                    return new StreamResource(
                            () -> {return DynamiccontentUI.class.getResourceAsStream(aLanguageChoice.getResourceName());}, 
                            aLanguageChoice.name() + "-language-flags.svg");
                });
09:44:08,243 WARNING [com.vaadin.server.ConnectorResourceHandler]
 (default task-10) ComboBox (14) did not handle connector request for null/italian-language-flags.svg