  The spectrum of massless Dirac fermions on the surface of a topological
insulator in a perpendicular magnetic field $B$ contains a $B$-independent
"zeroth Landau level", protected by chiral symmetry. If the Dirac equation is
discretized on a lattice by the method of "Wilson fermions", the chiral
symmetry is broken and the zeroth Landau level is broadened when $B$ has
spatial fluctuations. We show how this lattice artefact can be avoided starting
from an alternative nonlocal discretization scheme introduced by Stacey. A key
step is to spatially separate the states of opposite chirality in the zeroth
Landau level, by adjoining $+B$ and $-B$ regions.
