The speed of light in vacuum is 1 / sqrt(mu_0 e_0). In matter, this is modified using relative permittivity and permeability, so the speed of light in matter is c / sqrt(k_m k_e). In diamagnetic materials, isn't k_m < 1? Then we'd have light going faster than 3 * 10^8 m/s in, say, nitrogen gas, which is diamagnetic. What's wrong?