When looking at the docstring of `pad`: ``` In [21]: pd.Series.pad? Signature: pd.Series.pad( self: 'FrameOrSeries', axis=None, inplace: 'bool_t' = False, limit=None, downcast=None, ) -> 'Optional[FrameOrSeries]' Docstring: Synonym for :meth:`DataFrame.fillna` with ``method='ffill'``. Returns ------- {klass} or None Object with missing values filled or None if ``inplace=True``. ``` Or on the online version: https://pandas.pydata.org/docs/dev/reference/api/pandas.Series.pad.html There is a ``{klass}`` that has not been properly substituted.