In the following code snippet,  I get the error: pandas does not have the attribute ewma. I can see it the documentation. Any ideas?

import pandas as pd

class BetaAlgorithm(QCAlgorithm):
    def Initialize(self):
         ....

    def Rebalance(self):

        # Fetch the historical data to perform the linear regression
        history = self.History(
            self.symbols, 
            self.lookback,
            Resolution.Daily).close.unstack(level=0)
            

        prices = pd.ewma(history, span = self.lookback).as_matrix(self.symbols)
        

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