Odd merge error
```
In [6]: df1
Out[6]: 
                     x
1970-01-16 08:09:36  a

In [7]: df2
Out[7]: 
                     y
1970-01-16 08:09:36  b
1970-01-16 08:09:36  c

In [8]: merge(df1, df2)
---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
/home/wesm/code/pandas/<ipython-input-8-90d5dd612b4d> in <module>()
----> 1 merge(df1, df2)

/home/wesm/code/pandas/pandas/tools/merge.pyc in merge(left, right, how, on, left_on, right_on, left_index, right_index, sort, suffixes, copy)
     30                          right_index=right_index, sort=sort, suffixes=suffixes,
     31                          copy=copy)
---> 32     return op.get_result()
     33 if __debug__: merge.__doc__ = _merge_doc % '\nleft : DataFrame'
     34 

/home/wesm/code/pandas/pandas/tools/merge.pyc in get_result(self)
    174 
    175     def get_result(self):
--> 176         join_index, left_indexer, right_indexer = self._get_join_info()
    177 
    178         # this is a bit kludgy

/home/wesm/code/pandas/pandas/tools/merge.pyc in _get_join_info(self)
    247         else:
    248             # max groups = largest possible number of distinct groups
--> 249             left_key, right_key, max_groups = self._get_group_keys()
    250 
    251             join_func = _join_functions[self.how]

/home/wesm/code/pandas/pandas/tools/merge.pyc in _get_group_keys(self)
    411             group_sizes.append(count)
    412 
--> 413         left_group_key = get_group_index(left_labels, group_sizes)
    414         right_group_key = get_group_index(right_labels, group_sizes)
    415 

/home/wesm/code/pandas/pandas/core/groupby.pyc in get_group_index(label_list, shape)
   1921         return label_list[0]
   1922 
-> 1923     n = len(label_list[0])
   1924     group_index = np.zeros(n, dtype=np.int64)
   1925     mask = np.zeros(n, dtype=bool)

IndexError: list index out of range
```
