adict-guess-dictionary fails in org-mode with member-ignore-case: Wrong type argument: stringp, nil

[thblt]

On my computer (specs below), auto-dictionary-mode is unusable in org-mode.

Steps to reproduce (from emacs -Q)

1. Evaluate:

   (require 'package)
   (package-initialize)
   (require 'auto-dictionary)
   (require 'flyspell)

2. Open a new buffer, M-x org-mode

3. M-x flyspell-mode and M-x auto-dictionary-mode

4. Type some text in some language.

5. M-x adict-guess-dictionary

This fails with member-ignore-case: Wrong type argument: stringp, nil (in Messages, the part after ":" also appears in the modeline.

Repeating exactly the same steps in text-mode works perfectly.

Test configuration

• ArchLinux, up-to-date, on MacBook Air 2011.
• Emacs 25.1.1 from official repositories.
• Up-to-date packages downloaded by order of priority from Melpa stable, Gnu, Melpa unstable. Auto-dictionary itself is version 1.1.