public Array override(Array superior) {
            if (mode == ArrayMode.INDEX && superior.mode == ArrayMode.INDEX) {
                elements.putAll(superior.elements);
            } else {
                for (ConfigPayloadBuilder builder : superior.elements.values()) {
                    append().override(builder);
                }
            }
            return this;
        }
Try and verify that index mode is possible.
config/src/main/java/com/yahoo/vespa/config/ConfigPayloadBuilder.java