ApplyInPandas failing at a particular grouped item

[mbejarano89]

Hello,

I have a code that performs a forecast for 21k items in parallel. It looks like this:

 def forward_forecast(data):
    model = ETSModel(window_data, error='add', trend='add', seasonal=None)
                fitted_model = model.fit(disp=0)
                # Forecast the missing value
                forecast = fitted_model.forecast(steps=1).values[0]
                # Replace the missing value with the forecasted value
                data.loc[start_index:end_index-1, 'y_hat'] = forecast
                return data
result = data.groupBy("item").applyInPandas(foreward_forecast,schema)

When I run this code with a couple of items it run fine, but when I try using the 21k items, it fails at one item and gives this error: "unsupported operand type(s) for -: 'NoneType' and 'int'"

I am trying to figure out how to troubleshoot it and find  out at which item my function is failing.

Thanks