This can happen if a file is closed during unix socket GC. The same bug was fixed for devfs descriptors in r228361. PR: 242913 Reported and tested by: iz-rpi03@hs-karlsruhe.de Reviewed by: hselasky, kib MFC after: 1 week Sponsored by: The FreeBSD Foundation Differential Revision: https://reviews.freebsd.org/D23178