def _modify_fringe_params(adinputs, params):
    """
    This function modifies the param dictionary for the makeFringeForQA() and
    makeFringeFrame() primitives, to allow subtract_median_image=None to be
    passed.

    Parameters
    ----------
    adinputs: list
        AD instances being processed
    params: dict
        parameters passed to the calling primitive

    Returns
    -------
    dict: a (possibly modified) version of params
    """
    if params["subtract_median_image"] is None:
        params["subtract_median_image"] = any(ad.telescope() == "Gemini-South"
                                              for ad in adinputs)
    return params