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DataFrame.sum(self, axis=None, skipna=None, level=None, numeric_only=None, min_count=0, **kwargs)
min_count: int, default 0
The required number of valid values to
perform the operation. If fewer than min_count non-NA values are
present the result will be NA.New in version 0.22.0: Added with the default being 0. This means the
sum of an all-NA or empty Series is 0, and the product of an all-NA or
empty Series is 1.
Quoting from pandas latest docs it says the min_count will be 0 for the all-NA series.
If you say min_count=1 then the result of the sum will be a NaN.