serie.str.split() different from pd.Series.str.split(serie)
Hi,

I am learning Python and pandas.
Is this an expected behavior ?

```
serie= pd.Series(['a b', 'a b', 'a b', 'a b', 'a b', 'a b'])
serie_splited  =serie.str.split()
serie_splited  = pd.Series.str.split(serie)
```

serie.str.split() works correctly.

and pd.Series.str.split(serie) gets an error  : 
 
 File "D:\Dev2\Anaconda3\lib\site-packages\pandas\core\strings.py", line 160, in _map
    if not len(arr):

TypeError: len() of unsized object
