I am selecting a Universe of 20 stocks using the CoarseUniverse function. When I later try to get the price of one of these assets, I get the error 

The error was Exception : This asset symbol (AAPL R735QTJ8XC9X) was not found in your security list. Please add this security or check it exists before using it with 'Securities.ContainsKey("AAPL R735QTJ8XC9X")

What did I do wrong? Here is the complete code:

class Algorithm(QCAlgorithm): def Initialize(self): self.SetStartDate(2012,1,1) self.SetEndDate(2012,2,5) self.SetCash(100000) self.UniverseSettings.Resolution = Resolution.Daily self.AddUniverse(self.CoarseSelectionFunction) self.AddEquity('SPY', Resolution.Daily) self.Schedule.On(self.DateRules.MonthStart("SPY"), self.TimeRules.AfterMarketOpen("SPY", 60), Action(self.Rebalance)) def CoarseSelectionFunction(self, coarse): CoarseWithFundamental = [x for x in coarse if x.HasFundamentalData] sortedByDollarVolume = sorted(CoarseWithFundamental, key=lambda x: x.DollarVolume, reverse=True) self.universe = [x.Symbol for x in sortedByDollarVolume[:20]] return self.universe def Rebalance(self): self.Securities[self.universe[0]].Price