KeyNotFound-exception-on-trying-to-extract-LogReturn-from-BarHistory-Cache

CODE:

using WealthLab.Backtest;
using System;
using WealthLab.Core;
using WealthLab.Indicators;
using System.Collections.Generic;

namespace WealthScript7
{
   public class _Ask : UserStrategyBase
   {
      private LogReturn _LogReturn;

      public override void Initialize(BarHistory bars)
      {
         _LogReturn = new LogReturn(bars.Close, 20);
         bars.Cache["Return"] = _LogReturn;
         StartIndex = 20;
      }

      public override void PreExecute(DateTime dt, List<BarHistory> participants)
      {
         foreach (BarHistory bh in participants)
         {
            try
            {
               LogReturn _LogReturn = (LogReturn)bh.Cache["LogReturn"];
            }
            catch (Exception ex)
            {
               WriteToDebugLog(" Symbol = " + bh.Symbol + " Date = " + bh.GetDate(GetCurrentIndex(bh)) + " Exception = " + ex);
            }
         }
      }

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