def METHOD_NAME(self):
    exc = self.assert_mfd_error(
        TaperedGRMFD,
        min_mag=-1, max_mag=2, corner_mag=1.5, bin_width=0.4, a_val=1, 
        b_val=2
    )
    self.assertEqual(str(exc), 'minimum magnitude -1 must be non-negative')