Adding TradeVolumeIndex to existing cbot

Morning,

I am having trouble adding the indicator TradeVolumeIndex to one of my existing cbots, code snippet:

       #region Indicator declarations
        private SimpleMovingAverage sma1 { get; set; }
        private RelativeStrengthIndex rsi { get; set; }
        private TradeVolumeIndex tvi { get; set; }
        #endregion

        #region cTrader events
        protected override void OnStart()
        {
            // construct the indicators
            sma1 = Indicators.SimpleMovingAverage(SourceSma1, PeriodsSma1);
            rsi = Indicators.RelativeStrengthIndex(RSISource, RSIPeriods);
            tvi = Indicators.TradeVolumeIndex;
        }
Getting the following error message on the construct for the  TradeVolumeIndex indicator, not sure I understand this.     
Error CS0428: Cannot convert method group 'TradeVolumeIndex' to non-delegate type 'cAlgo.API.Indicators.TradeVolumeIndex'. Did you intend to invoke the method?

This is an example of how I wish to use the indicator.

 if (tvi.Result.IsFalling())
            {
                ClosePosition(TradeType.Buy);
            }

Much appreciate if someone could give me some of their experience on this issue.

Many thank

Brian