Error in post-command-hook (completion-in-region--postch): (wrong-type-argument stringp nil)

[Tassilo Horn <tsdh@gnu.org>]

Hi all,

I'm writing a completion at point function for nREPL (which is about the
same as SLIME/Swank is for Common Lisp).  However, if my function is in
`completion-at-point-functions' I frequently get the error in the
subject, and as a annoying result `completion-in-region-mode' isn't
toggled off anymore.

I've read the docs about completion at point several times, and my
function looks good to me.  It'll always return

  (START END (LIST-OF-STRINGS))

with valid positions and the completions really being strings, or nil if
there's no possible completion.

--8<---------------cut here---------------start------------->8---
(defun nrepl-complete-at-point ()
  (interactive)
  (let ((sap (symbol-at-point)))
    (when (and sap (not (in-string-p)))
      (nrepl-send-string "(require 'complete.core)" "user" 'identity)
      (let ((form (format "(complete.core/completions \"%s\" *ns*)" sap))
	    (bounds (bounds-of-thing-at-point 'symbol)))
	(let ((completions (car (read-from-string
				 (plist-get (nrepl-send-string-sync form nrepl-buffer-ns)
					    :value)))))
	  (when completions
	    ;; TODO: Remove me again!
	    (assert (null (remove-if 'stringp completions)))
	    (list (car bounds) (cdr bounds) completions)))))))
--8<---------------cut here---------------end--------------->8---


Looking at the definition of `completion-in-region--postch' the only
place where the error might be triggered is in the

  (funcall completion-in-region-mode--predicate)

line.  The value of that is

--8<---------------cut here---------------start------------->8---
completion-in-region-mode--predicate is a variable defined in `minibuffer.el'.
Its value is #[0 "\301 \242\300=\207"
    [9284 nrepl-complete-at-point]
    2 "\n\n(fn)"]
--8<---------------cut here---------------end--------------->8---

There are no other function in `completion-at-point-functions'.

I'm happy for any pointers!

Bye,
Tassilo