Pandas Series.add(fill_value=0) changes int dtype to float

Code Sample

import pandas as pd
pd.Series([1,2,3]).add(pd.Series([100]), fill_value=0)

Problem description

The resulting dtype should be int (as the inputs and fill_value are all ints), but it is float:

0    101.0
1      2.0
2      3.0
dtype: float64

Output of pd.show_versions()

python: 3.5.4.final.0 python-bits: 64 pandas: 0.20.3 numpy: 1.13.1

Comment From: jreback

aligned causes a shift to float64. We don't upcast automatically on fillna, becuase of perf (it can be non-cheap). So this is an unavoidable side affect of lack of good NA support in numpy.

In [1]: pd.Series([1,2,3]).align(pd.Series([100]))
Out[1]: 
(0    1
 1    2
 2    3
 dtype: int64, 0    100.0
 1      NaN
 2      NaN
 dtype: float64)