groupby on single col df results in ValueError on Master, works on 0.18.1
On 0.18.1 the following works without error

```
df = pd.DataFrame({'a':list('abssbab')})
df.groupby('a').count()

```

this results in an empty df:

```
Out[255]:
Empty DataFrame
Columns: []
Index: [a, b, s]
```

On Master it results in an error see related: https://github.com/pydata/pandas/issues/13393
