Flutter RangeLabels on RangeSlider not working

Here's how you can achieve the same

SliderTheme(
    data: SliderThemeData(
      showValueIndicator: ShowValueIndicator.always
    ),
    child: RangeSlider(
      values: _values,
      min: 0,
      max: 1000,
      labels: RangeLabels('${_values.start.round()}', '${_values.end.round()}'),
      inactiveColor: Colors.grey,
      activeColor: Colors.black,
      onChanged: (RangeValues values) {
        setState(() {
          _values = values;
        });
      },
    ),
);

Here _values are the RangeValues defined in State class like this

     class _FilterScreenState extends State<FilterScreen>{
      RangeValues _values = RangeValues(100, 900);
}