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=0.2
    )
    self.assertEqual(str(exc), 'bin width -0.4 must be positive')