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