We derive an upper bound on the reliability function of mismatched decoding for zero-rate codes. The bound is based on a result by Komlos that shows the existence of a subcode with certain symmetry properties. The bound is shown to coincide with the expurgated exponent at rate zero for a broad family of channel and decoding metric pairs.

A full version of this paper is accessible at: https://arxiv.org/pdf/2101.10238.pdf