I have a problem getting vix and vxx historical data 

def Initialize(self):  

   self.quandlCodeVIX = "CBOE/VIX"

   self.quandlCodeVXV = "CBOE/VXV"

def OnData(self, slice):

  VIX_history_tradebars = self.History(self.quandlCodeVIX, 1, Resolution.Daily)

  VXV_history_tradebars = self.History(self.quandlCodeVXV, 1, Resolution.Daily)
  ####Above the problem occurs.

class QuandlVIX(Quandl):
    def __init__(self):
            self.ValueColumnName = "VIX CLOSE"

class QuandlVXV(PythonQuandl):
    def __init__(self):
        self.ValueColumnName = "CLOSE"

VIX_history_tradebars doesn't return me anything. I would like to get previous close price for VIX and VXV.  with this method. I could get current close price. But how can I get the previous data closing price ? Please help out