Unexpected results on groupby([]).sum()
#### Code Sample: 
```python
# n = df1.groupby(['Year', 'State', 'Regulator', 'Industry','Product', 'Count']).sum()     # <-- this produces the error

```
#### Problem description

[Hi, I think there's a problem dropping/excluding data points with groupby.sum function. I've performed the following code (see above), which at hindsight seemed ok until I compared with the same data using Excel and/or simple plot of the dataset. In addition, removing 'Count' will throw off values on other df columns. Thanks for checking this out.]

#### Expected Output

Year | 2012
State | **Alabama**
Regulator | **SEC**
Insurance/Annuity Products | 2
Stocks | 4


Year | 2012
State | **Alabama**
Regulator | **FDIC**
Debit Card | 1
Residential Mortgage | 3


#### Output of ``pd.df``

Year | 2012
State | **Alabama**
Regulator | **FDIC**
Debit Card | 1
Residential Mortgage | 1
