def join_stem_to_base_factors(df_stem, df_base_factors):
    """Join formatted stem section to combined Base Premium and Factors columns"""
    df_formatted = df_stem.merge(
        df_base_factors,
        how='left', left_index=True, right_index=True
    ).fillna(0.)  # The only mising values are from 'error' rows
    return df_formatted