error with groupby and transform while using datetime64[ns] as part of a key
from pandas.util.testing import choice
from pandas import DataFrame, date_range
from numpy.random import randn

n = 8
dates = date_range('1/1/2000', periods=n, freq='1T')
colors = choice(['red', 'green'], size=n)

df = DataFrame({'value': randn(n), 'date': dates, 'color': colors})

zscore = lambda x: (x - x.mean()) / x.std()

df
Out[2]: 
   color                date     value
0    red 2000-01-01 00:00:00 -0.382902
1  green 2000-01-01 00:01:00  0.991723
2  green 2000-01-01 00:02:00 -0.442476
3  green 2000-01-01 00:03:00 -0.776372
4    red 2000-01-01 00:04:00 -1.372511
5    red 2000-01-01 00:05:00 -0.276006
6    red 2000-01-01 00:06:00 -0.182162
7    red 2000-01-01 00:07:00 -0.232967

[8 rows x 3 columns]

df.dtypes
Out[3]: 
color            object
date     datetime64[ns]
value           float64
dtype: object

df.groupby(['color','date'])['value'].transform(zscore)

---

KeyError                                  Traceback (most recent call last)
<ipython-input-4-97e715db56f2> in <module>()
----> 1 df.groupby(['color','date'])['value'].transform(zscore)

C:\Users\MD\Anaconda\lib\site-packages\pandas-0.13.1.dev-py2.7-win-amd64.egg\pandas\core\groupby.pyc in transform(self, func, _args, *_kwargs)
   1845             # this needs to be an ndarray
   1846             result = Series(result)
-> 1847             result.iloc[self._get_index(name)] = res
   1848             result = result.values
   1849 

C:\Users\MD\Anaconda\lib\site-packages\pandas-0.13.1.dev-py2.7-win-amd64.egg\pandas\core\groupby.pyc in _get_index(self, name)
    265         """ safe get index """
    266         try:
--> 267             return self.indices[name]
    268         except:
    269             if isinstance(name, Timestamp):

KeyError: ('green', Timestamp('2000-01-01 00:01:00', tz=None))
