I have created a Data frame in Ipython notebook in which there are some missing values and used it for the calculation of column sum of a Data frame but I am not getting any missing value in the result.I want to know how to get the missing value in the column sum of a Data frame.

```
df = DataFrame([[1.4, np.nan], [7.1, -4.5],
.....: [np.nan, np.nan], [0.75, -1.3]],
.....: index=['a', 'b', 'c', 'd'],
.....: columns=['one', 'two'])
df
one two
a 1.40 NaN
b 7.10 -4.5
c NaN NaN
d 0.75 -1.3
df.sum()
one 9.25
two -5.80
```