[Python] statsmodel.SARIMAX .forecast() method do not execute

I am trying to use SARIMAX model for TS forecasting. However, I got some kind of error i dont know how to handle. My code is simple:

import statsmodels.api as sm

fit = sm.tsa.statespace.SARIMAX(train).fit()
sarima = fit.forecast()


train data looks like

y
ds
2015-01-07 1
2015-01-14 64
2015-01-21 16
2015-01-28 50
2015-02-04 7


I got the error

/usr/local/lib/python3.6/dist-packages/statsmodels/tsa/base/datetools.py in
_date_from_idx(d1, idx, freq)
84 offset. For now, this needs to be taken care of before you get
here.
85 """
---> 86 return _maybe_convert_period(d1) + int(idx) *
_freq_to_pandas[freq]
87
88

TypeError: unsupported operand type(s) for *: 'int' and 'NoneType'


Any ideas what I am doing wrong?

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