Cannot model losses in RF module

Hi

I've been trying to model a lossy dielectric in the RF module, using the transient electromagnetic waves interface. However, if I set the model to be based on the refractive index, COMSOL appears to ignore the imaginary part in my material definitions. If I then try and model the losses by introducing an imaginary component to the relative permittivity, COMSOL always treats it like a gain medium regardless of the sign of the imaginary component.

Is there anything I can do to fix this?

Thanks.