GroupBy bug with time series
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``````
a = pd.DataFrame({'dt':pd.datetime(2012,4,25,9,30,0,393000),'size':50,'price':87.67},index=[pd.datetime(2012,4,25,9,30,0,393000)])
dt = a.index[0]
minutely = pd.date_range(dt,dt,freq='5T')
grouped = a.groupby(minutely.asof)
grouped['size'].last()

# error: len() of unsized object```
``````
