An electromagnetic field in a photonic crystal is described by a hermitian eigenvalue problem with an overlap matrix which may not be positive definite if the system is composed of negative index media. Then the system is mapped to a non-hermitian system with symmetry restriction. It implies the existence of exceptional lines in the 2-dimensional Brillouin zone. Physically it implies the disappearance of photonic bands (propagating modes) beyond some momentum lines. The paper “A symmetry-protected exceptional ring in a photonic crystal with negative index media” by Takuma Isobe, Tsuneya Yoshida and Yasuhiro Hatsugai has been published in Nanophotonics, on June 12 (2023). See also arXiv:2212.11090.