  Dimensional reduction of the M5 brane on a Lorentzian manifold along a
lightlike direction results in a five-dimensional gauge theory, which can be
reformulated covariantly in six dimensions, where one puts the Lie derivatives
along the lightlike direction of all fields to zero as constraints. Without
imposing these constraints, we have a nonsupersymmetric six-dimensional gauge
theory that we may expect shall have a six dimensional gauge symmetry. However
this gauge symmetry has an anomaly for certain Lorentzian six-manifolds. We
show that this gauge anomaly can be canceled by adding a WZW theory in the 2d
space that is spanned by two lightlike directions.
