def METHOD_NAME(self):
    for b_val in [-2, 0]:
        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=b_val
        )
        self.assertEqual(str(exc),
                         'b-value %g must be non-negative' % b_val)