Completion after a close paren signals an error.

[Malabarba]

If I place point after (inc 1), and try to trigger completion (tab, in my case) I get the following error.

Debugger entered--Lisp error: (error "Marker does not point anywhere")
  completion-in-region(#<marker in no buffer> nil #[771 "\211\242\302=\206\n�\211\303=?\2053�r\301\204��p\202(�\304 \305�!\203%�\306�!\202&�p\262�q\210\307�\300�!��$)\207" [cider-complete nil boundaries metadata minibuffer-selected-window window-live-p window-buffer complete-with-action] 8 "\n\n(fn STRING PRED ACTION)"] nil)
  completion-at-point()
  indent-for-tab-command(nil)
  funcall-interactively(indent-for-tab-command nil)
  call-interactively(indent-for-tab-command nil nil)
  command-execute(indent-for-tab-command)

It seems to me this is because the following form inside the completion-at-point function

(run-hook-wrapped 'completion-at-point-functions
                  #'completion--capf-wrapper 'all)

returns a value like this (cider-complete-at-point nil nil ...). The elements 1 and 2 on this list are nil, but Emacs expects them to be integers or markers, and so the error is signaled.
IIRC, in a situation where CIDER can't do completion, then it should simply return nil, instead of returning an invalid list.