Fillna a categories series with another series breaks (original) (raw)

On pandas 0.18.1, for some values of a, b

Code Sample

pd.Categorical([1,2,None, None]).fillna(pd.Series([1,1, a, b]))

Expected Output

Either the same as

pd.Categorical([1, 2, a, b])

or

ValueError: fill value must be in categories

If a or b do not match the categories. This is what whe get when we do pd.Categorical([1, 2, None, None]).fillna(3)

Actual output

ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()
This error message seems rather unenlightening...