def METHOD_NAME(monkeypatch):
    MOCK_UCONV_MUL_FACS = pd.DataFrame(
        [
            ["concso4", "ug S/m3", "ug m-3", 1],
            ["concso4", "ug S/m3", "ug m-3", 2],
        ],
        columns=["var_name", "from", "to", "fac"],
    ).set_index(["var_name", "from"])
    monkeypatch.setattr("pyaerocom.units_helpers.UCONV_MUL_FACS", MOCK_UCONV_MUL_FACS)
    with pytest.raises(UnitConversionError) as e:
        _unit_conversion_fac_custom("concso4", "ug S/m3")
    assert "Could not find unique conversion factor in table" in str(e.value)