When I try to exit crontab-edit using C-x C-s I get:
Signaling: (wrong-type-argument stringp nil)
intern(nil [0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 ~/\.crontab 0])
vc-file-getprop(nil vc-backend)
vc-after-save()
basic-save-buffer()
save-buffer(1)
call-interactively(save-buffer)