def coerce_dtype(self, series_or_index: pd.Index) -> pd.Index:
"""Coerce type of a pd.Index by type specified in pandas_dtype.
:param pd.Index series: One-dimensional ndarray with axis labels
(including time series).
:returns: ``Index`` with coerced data type
"""
if self._pandas_dtype is PandasDtype.String:
return series_or_index.where(series_or_index.isna(),
series_or_index.astype(str))
# only coerce non-null elements to string
return series_or_index.astype(self.dtype)
