PreExecute-exception-Index-was-out-of-range

CODE:

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

namespace WealthScript1
{
   public class My : UserStrategyBase
   {
      private static List<BarHistory> Buys = new List<BarHistory>();

      public override void Initialize(BarHistory bars)
      {
         StartIndex = 101;
         _Mom = new Momentum(bars.Close, 100);
         bars.Cache["Momentum"] = _Mom;
      }

      public override void PreExecute(DateTime dt, List<BarHistory> participants)
      {
         foreach (BarHistory bh in participants)
         {
            Momentum _M = (Momentum)bh.Cache["Momentum"];
            int idx = GetCurrentIndex(bh);
            double MomVal = _Mom[idx];
            bh.UserData = MomVal;
         }
         participants.Sort((a, b) => a.UserDataAsDouble.CompareTo(b.UserDataAsDouble));

         Buys.Clear();
         for (int n = 0; n <= 19; n++)
         {
            if (n >= participants.Count)
               break;
            Buys.Add(participants[n]);
         }
      }

      public override void Execute(BarHistory bars, int idx)
      {
         bool inBuyList = Buys.Contains(bars);

         if (!HasOpenPosition(bars, PositionType.Long))
         {
            if (inBuyList)
               PlaceTrade(bars, TransactionType.Buy, OrderType.Market);
         }
         else
         {
            if (!inBuyList)
               PlaceTrade(bars, TransactionType.Sell, OrderType.Market);
         }
      }
      private Momentum _Mom;
   }
}