Hi! I got the following error *ValueError: optimizer got an empty parameter list* when I run the code containing the following classes:

```
class Mass_and_v():
def __init__(self, param):
self.parameter = param
# get_mass + v_circ_tot
def __call__(self, x):
ener_f = 56.0 # keV
r, mass = rar.model(self.parameter[3:].detach().numpy(),ener_f)
isnan = np.argwhere(np.isnan(mass))
if (np.any(isnan)):
np.argwhere(np.isnan(mass))[0][0]
else:
k = -1
r_s = r[0:k]
mass_s = mass[0:k]
mass_spline = InterpolatedUnivariateSpline(r_s, mass_s, k=4)
r_max = np.amax(r_s)
if x <= r_max:
return np.sqrt(v_circ_b(x,self.parameter[0].detach().numpy())**2 + v_circ_MN(x,self.parameter[1:3].detach().numpy())**2 + G_u*mass_spline(x)/x)
else:
return np.sqrt(v_circ_b(x,self.parameter[0].detach().numpy())**2 + v_circ_MN(x,self.parameter[1:3].detach().numpy())**2 + G_u*mass_spline(r_max)/x)
class FitModel(nn.Module):
def __init__(self):
super(FitModel, self).__init__()
self.nonlinear_stack = Mass_and_v(param_t)
def forward(self, x):
output = self.nonlinear_stack(x)
return output
```

I got the error when the code comes to the line

```
optimizer = torch.optim.Adam(model.parameters(), lr=eta, eps=1e-08, weight_decay=0, amsgrad=False)
```

I hope someone can help me. Thank you so much in advance