  Br\'ee, Demircan, and Steinmeyer [Phys. Rev. Lett. 106, 183902 (2011)]
recently calculated higher-order Kerr effect (HOKE) coefficients using the
nonlinear Kramers-Kronig relations. We show that the saturated and negative
nonlinear index of refraction obtained by Br\'ee et al. is an artifact of their
use of a perturbative theory of ionization in an intensity region where it
breaks down.

Optics;Atomic Physics;Plasma Physics