test_pivot failed with KeyError: nan

[anmyachev]

Found in #6954

Traceback:

    def test_pivot(data, index, columns, values):
>       eval_general(
            *create_test_dfs(data),
            lambda df, *args, **kwargs: df.pivot(*args, **kwargs),
            index=index,
            columns=columns,
            values=values,
        )

modin\pandas\test\dataframe\test_default.py:602:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
modin\pandas\test\utils.py:937: in eval_general
    comparator(*values, **(comparator_kwargs or {}))
modin\pandas\test\utils.py:709: in df_equals
    assert_dtypes_equal(df1, df2)
modin\pandas\test\utils.py:635: in assert_dtypes_equal
    if isinstance(df1.dtypes, (pandas.Series, pd.Series)):
modin\pandas\base.py:4193: in __getattribute__
    attr = super().__getattribute__(item)
modin\pandas\dataframe.py:328: in dtypes
    return self._query_compiler.dtypes
modin\core\storage_formats\pandas\query_compiler.py:321: in dtypes
    return self._modin_frame.dtypes
modin\core\dataframe\pandas\dataframe\dataframe.py:388: in dtypes
    dtypes = self._dtypes.get()
modin\core\dataframe\pandas\metadata\dtypes.py:936: in get
    self._value = self._value.to_series()
modin\core\dataframe\pandas\metadata\dtypes.py:441: in to_series
    self.materialize()
modin\core\dataframe\pandas\metadata\dtypes.py:424: in materialize
    self._known_dtypes = {
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

.0 = <range_iterator object at 0x00000288A87CA150>

    self._known_dtypes = {
>       self.columns_order[i]: self._known_dtypes[self.columns_order[i]]
        for i in range(len(self.columns_order))
    }
E   KeyError: nan

modin\core\dataframe\pandas\metadata\dtypes.py:425: KeyError