dataframe slice assignment failure
```
In [27]: df
Out[27]: 
   a         b         c   d
0  2  0.103795 -0.066523 NaN
1  3 -0.939895 -0.208760 NaN
2  9  1.968456  0.419374 NaN
3  6 -0.105170  0.162064 NaN
4  7  0.381707  2.126133 NaN

In [28]: df[df['a']==3]
Out[28]: 
   a         b        c   d
1  3 -0.939895 -0.20876 NaN

In [29]: df[df['a']==2]
Out[29]: 
   a         b         c   d
0  2  0.103795 -0.066523 NaN

In [30]: df[df['a']==2] = 100
---------------------------------------------------------------------------
IndexingError                             Traceback (most recent call last)
/home/adam/code/pandas/<ipython-input-30-f7da2453a9f6> in <module>()
----> 1 df[df['a']==2] = 100

/home/adam/code/pandas/pandas/core/frame.py in __setitem__(self, key, value)
   1491             self._boolean_set(key, value)
   1492         elif isinstance(key, (np.ndarray, list)):
-> 1493             return self._set_item_multiple(key, value)
   1494         else:
   1495             # set column


/home/adam/code/pandas/pandas/core/frame.py in _set_item_multiple(self, keys, value)
   1516                 self[k1] = value[k2]
   1517         else:
-> 1518             self.ix[:, keys] = value
   1519 
   1520     def _set_item(self, key, value):

/home/adam/code/pandas/pandas/core/indexing.py in __setitem__(self, key, value)
     59                 raise IndexingError('only tuples of length <= %d supported',
     60                                     self.ndim)
---> 61             indexer = self._convert_tuple(key)
     62         else:
     63             indexer = self._convert_to_indexer(key)

/home/adam/code/pandas/pandas/core/indexing.py in _convert_tuple(self, key)
     68         keyidx = []
     69         for i, k in enumerate(key):
---> 70             idx = self._convert_to_indexer(k, axis=i)
     71             keyidx.append(idx)
     72         return tuple(keyidx)

/home/adam/code/pandas/pandas/core/indexing.py in _convert_to_indexer(self, obj, axis)
    282         elif _is_list_like(obj):
    283             if com._is_bool_indexer(obj):
--> 284                 objarr = _check_bool_indexer(labels, obj)
    285                 return objarr
    286             else:

/home/adam/code/pandas/pandas/core/indexing.py in _check_bool_indexer(ax, key)
    417         mask = com.isnull(result)
    418         if mask.any():
--> 419             raise IndexingError('cannot index with vector containing '
    420                                 'NA / NaN values')                                                                                   
    421 

IndexingError: cannot index with vector containing NA / NaN values
```
