SeriesGroupBy.count() failing with TypeError
``` python
>>> import pandas as pd
>>> pd.__version__
u'0.17.0rc1'

>>> df = pd.DataFrame({'A': [0, 0, 1, 1], 'B': range(4)})
>>> df.groupby('A').count()['B']
A
0    2
1    2
Name: B, dtype: int64

>>> df.groupby('A')['B'].count()
Traceback (most recent call last):
...
TypeError: Cannot cast array data from dtype('int64') to dtype('int32') according to the rule 'safe'
```
