Fitting creates active material

[alec-flexcompute]

Describe the bug
When fitting 3 refractive index values, I get a material that has a negative k value, and diverges.

To Reproduce
To reproduce, see the attached example notebook:

[momchil-flex]

Thanks @alec-flexcompute . Could you post code snippets rather than images so it's easy to just copy paste.

[alec-flexcompute]

import numpy as np
import tidy3d as td

wvls = np.array([275e-3, 260e-3, 255e-3])
n_nAlGaN = np.array([2.72, 2.68, 2.53])

from tidy3d.plugins.dispersion import FastDispersionFitter, AdvancedFastFitterParam

nAlGaN_fitter = FastDispersionFitter(wvl_um=wvls, n_data=n_nAlGaN)
nAlGaN_mat, _ = nAlGaN_fitter.fit()

freq_list = nAlGaN_mat.angular_freq_to_Hz(nAlGaN_mat._imag_ep_extrema_with_samples())
ep = nAlGaN_mat.eps_model(freq_list)
for e in ep:
    print(e.imag)