  We show that a large class of null electromagnetic fields are immune to any
modifications of Maxwell's equations in the form of arbitrary powers and
derivatives of the field strength. These are thus exact solutions to virtually
any generalized classical electrodynamics containing both non-linear terms and
higher derivatives, including, e.g., non-linear electrodynamics as well as QED-
and string-motivated effective theories. This result holds not only in a flat
or (anti-)de Sitter background, but also in a larger subset of Kundt
spacetimes, which allow for the presence of aligned gravitational waves and
pure radiation.
