RL-GO in Plane Shell Not Allowed with Copper Shell

Is it possible to use the ray-launching geometric optics method with lossy materials? We are trying to model RF propagation in an empty airplane shell (consisting only of faces) but receive the following error when we change the shell material from PEC to copper with a thickness of 1 mm: ERROR 33858: Metallic triangles for ray-launching GO found with unsupported losses.

Is there any way to work around this error (in FEKO Suite 7.0)? I cannot post the plane model, but the shell can be thought of as an arbitrary closed surface inside of which there is a dipole antenna source. We would like to make the metallic shell have a thickness so to eliminate fields being generated external to the airplane with PEC as the shell material.