Are you saying that Emacs 26 doesn't signal a similar error in this
case? It does for me.
No, I did not imply that.
I just tried out on 26.1, and I get a similar error (integerp instead of fixnump):
Debugger entered--Lisp error: (wrong-type-argument integerp nil)
encode-time(nil nil nil 5 9 2018 nil nil nil)
apply(encode-time (nil nil nil 5 9 2018 nil nil nil))
eval((apply (function encode-time) (parse-time-string "2018-09-05")) nil)
elisp--eval-last-sexp(nil)
eval-last-sexp(nil)
funcall-interactively(eval-last-sexp nil)
call-interactively(eval-last-sexp nil nil)
command-execute(eval-last-sexp)
So.. is this a user error.. that elisp is not valid (i.e. nil not allowed in that list)?