The constructor CompositeDataSupport(CompositeType, Map) ends up calling a private method makeMap(Map<String,?> items) which does this:

        if (items.containsKey(null) || items.containsKey(""))
            throw new IllegalArgumentException("Null or empty item name");

If items is a Map that does not support null keys then items.containsKey(null) is allowed to throw NullPointerException, which will mask the IllegalArgumentException. TreeMap is such a Map.