Bug? If so, suggestions on a work around?

I have the following:

```
df1['DecisionDate'].head()
Out[238]: 
ID
RED            2017-02-13 00:00:00
GREEN          2016-07-29 00:00:00
ORANGE         2017-01-26 00:00:00
PURPLE         2016-10-31 00:00:00
YELLOW          NaT
Name: DecisionDate, dtype: datetime64[ns]

```

And this:

```
df2['DecisionDate']
Out[239]: 
YELLOW   2014-04-05 00:00:00
Name: DecisionDate, dtype: datetime64[ns]

```

Now if I try to do this:

```
for ID in df2.index:

    df1.ix[ID,'DecisionDate'] = df2.ix[ID,'DecisionDate']

```

I get this

```
TypeError: long() argument must be a string or a number, not 'Timestamp'

```

No matter what I try I cannot seem to replace the `pd.NaT` with a timestamp value.

All values in both dataframe are timestamps. df2 observations are a subset of df1 observations so all values in df2.index are in df1.index.

Am I overlooking something straightforwards? Or is this a bug?

Thanks in advance for your help.

EDIT

Here is the full traceback. I'm not good at reading this but maybe it will help in diagnosing:

```
  File "C:\Users\Anaconda2\lib\site-packages\pandas\core\indexing.py", line 141, in __setitem__
    def _slice(self, obj, axis=0, kind=None):

  File "C:\Users\Anaconda2\lib\site-packages\pandas\core\indexing.py", line 533, in _setitem_with_indexer

  File "C:\Users\Anaconda2\lib\site-packages\pandas\core\indexing.py", line 473, in setter
    value = getattr(value, 'values', value).ravel()

  File "C:\Users\Anaconda2\lib\site-packages\pandas\core\internals.py", line 3168, in setitem

  File "C:\Users\Anaconda2\lib\site-packages\pandas\core\internals.py", line 3056, in apply
    align_copy = False

  File "C:\Users\Anaconda2\lib\site-packages\pandas\core\internals.py", line 668, in setitem
    def _replace_single(self, *args, **kwargs):

  File "C:\Users\Anaconda2\lib\site-packages\pandas\core\internals.py", line 2265, in _try_coerce_args
    Parameters

TypeError: long() argument must be a string or a number, not 'Timestamp'

```