def METHOD_NAME(
    categorical_fields: Optional[Union[str, List[str]]], numerical_fields: Optional[Union[str, List[str]]]
):
    if categorical_fields is None and numerical_fields is None:
        raise RuntimeError("Both `categorical_fields` and `numerical_fields` are None!")
    categorical_fields = categorical_fields or []
    numerical_fields = numerical_fields or []
    if not isinstance(categorical_fields, list):
        categorical_fields = [categorical_fields]
    if not isinstance(numerical_fields, list):
        numerical_fields = [numerical_fields]
    return categorical_fields, numerical_fields