BUG: groupby.transform confused about index

[dsm054]

Under some circumstances, transform is getting confused about what the output index should be:

>>> df = pd.DataFrame({"c1": [1], "c2": [2]})
>>> df
   c1  c2
0   1   2
>>> df.groupby('c1').transform(sum)
   c2
0 NaN
1   2

I haven't chased down why, but since we have

>>> df.groupby('c1').transform(lambda x: sum(x))
   c2
0   2

it must be another fast-path problem, like #9697.