Erro ao incluir 'elemento_md em .loc

Fui seguindo conforme a aula, mas o ao colocar o elemento_md dentro do [] do loc ele retorna um erro.

n = notas_fulano.shape[0]
n
7

elemento_md= (n+1)/2
elemento_md
4.0

notas_fulano.loc[elemento_md - 1]

---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
/usr/local/lib/python3.7/dist-packages/pandas/core/indexes/base.py in get_loc(self, key, method, tolerance)
   2897             try:
-> 2898                 return self._engine.get_loc(casted_key)
   2899             except KeyError as err:

pandas/_libs/index.pyx in pandas._libs.index.IndexEngine.get_loc()

pandas/_libs/index.pyx in pandas._libs.index.IndexEngine.get_loc()

pandas/_libs/hashtable_class_helper.pxi in pandas._libs.hashtable.PyObjectHashTable.get_item()

pandas/_libs/hashtable_class_helper.pxi in pandas._libs.hashtable.PyObjectHashTable.get_item()

KeyError: 3

The above exception was the direct cause of the following exception:

KeyError                                  Traceback (most recent call last)
5 frames
/usr/local/lib/python3.7/dist-packages/pandas/core/indexes/base.py in get_loc(self, key, method, tolerance)
   2898                 return self._engine.get_loc(casted_key)
   2899             except KeyError as err:
-> 2900                 raise KeyError(key) from err
   2901 
   2902         if tolerance is not None:

KeyError: 3.0

Qual a causa deste erro ?