Панды: pandas._libs.hashtable.Int64HashTable.get_item

У меня есть следующий код, работающий с фреймом данных df:

  print df
  categories = df['my_classification'].unique()
  for c in categories:
    print c
    win = df[df.result == 'Won'][df['my_classification'] == c]['prob'][0]

    print type(win)
    lost = df[df.result == 'Lost'][df['my_classification'] == c]['prob'][0]
    print type(lost)

Тогда я получил следующий результат:

   result          my_classification      prob
0  Won                   ENTERPRISE      0.657895
1  Won                   COMMERCIAL      0.342105
2  Lost                  ENTERPRISE      0.611842
3  Lost                  COMMERCIAL      0.388158
ENTERPRISE
<type 'numpy.float64'>

И ошибки:

There was a problem running this cell
KeyError 0 
KeyErrorTraceback (most recent call last)
<ipython-input-4-38a901f9868a> in <module>()
     38 
     39     print type(win)
---> 40     lost = df[df.result == 'Lost'][df['my_classification'] == c]['prob'][0]
     41 
     42     print type(lost)

/opt/conda/envs/python2/lib/python2.7/site-packages/pandas/core/series.pyc in __getitem__(self, key)
    599         key = com._apply_if_callable(key, self)
    600         try:
--> 601             result = self.index.get_value(self, key)
    602 
    603             if not is_scalar(result):

/opt/conda/envs/python2/lib/python2.7/site-packages/pandas/core/indexes/base.pyc in get_value(self, series, key)
   2426         try:
   2427             return self._engine.get_value(s, k,
-> 2428                                           tz=getattr(series.dtype, 'tz', None))
   2429         except KeyError as e1:
   2430             if len(self) > 0 and self.inferred_type in ['integer', 'boolean']:

pandas/_libs/index.pyx in pandas._libs.index.IndexEngine.get_value (pandas/_libs/index.c:4363)()

pandas/_libs/index.pyx in pandas._libs.index.IndexEngine.get_value (pandas/_libs/index.c:4046)()

pandas/_libs/index.pyx in pandas._libs.index.IndexEngine.get_loc (pandas/_libs/index.c:5085)()

pandas/_libs/hashtable_class_helper.pxi in pandas._libs.hashtable.Int64HashTable.get_item (pandas/_libs/hashtable.c:13913)()

pandas/_libs/hashtable_class_helper.pxi in pandas._libs.hashtable.Int64HashTable.get_item (pandas/_libs/hashtable.c:13857)()

KeyError: 0

Вот чего я не понимаю: выигрыш и проигрыш имеют один и тот же формат, почему win был в порядке, но lost выдал ошибку? Спасибо!