SMMA-indicator-produces-NaN-output-for-LRSlope-input

The SMMA indicator produces no output (i.e. NaN) when the input to it is from certain indicators such as LRSlope. The last plot in the code below produces nothing. I'm running WL8 Build 47.

CODE:
using WealthLab.Backtest;
using WealthLab.Core;
using WealthLab.Indicators;

namespace WealthScript2 
{
   public class PolyScreener : UserStrategyBase
   {
      public override void Initialize(BarHistory bars)
      {
         IndicatorBase smma = new SMMA(bars.Close, 5);
         PlotIndicatorLine(smma, WLColor.YellowGreen); //works

         IndicatorBase slope = new LRSlope(bars.Close, 4);
         IndicatorBase smmaSlope = new SMMA(slope, 5);
         PlotIndicatorLine(smmaSlope, WLColor.Green); //fails
      }

      public override void Execute(BarHistory bars, int idx) { }
   }
}