error msg MPOReturnsForecast about in MultiPeriodOptimization.ipynb

[scriptzza]

TypeErrorTraceback (most recent call last) in () 8 all_return_estimates[(t,tp1)]=return_estimate.loc[tp1] 9 ---> 10 returns_forecast = cp.MPOReturnsForecast(all_return_estimates) 11 results_MPO={}

TypeError: Can't instantiate abstract class MPOReturnsForecast with abstract methods weight_expr