The presence of a whistler wave instability (WWI) excited by runaway electrons may be the reason for the observation that the number of runaway electrons produced during disruptions in large tokamaks depends sensitively on the magnetic field strength. Previous work has shown that the linear growth rates of these waves are such that they are stable for high magnetic field (so the runaway beam can form) but unstable for low magnetic field. Here, it is shown that the quasi-linear diffusion process due to the WWI represents a very efficient pitch-angle scattering mechanism for runaways and consequently may stop runaway beam formation in large tokamak disruptions.