I have the following error every single time I press RET in org-mode : wrong-type-argument char-or-string-p nil. It's usually ok but it's bothering when it breaks a tag at the end of a line for example.

Here is the complete stack trace :

Debugger entered--Lisp error: (wrong-type-argument char-or-string-p nil)
call-interactively(org-return nil nil)

I'm using a GUI Emacs 25.2.1 (Org mode 8.2.10) on Windows 10

Any idea to fix it ? I've made several searchs on these functions in the doc but without success. Thanks for your insight.

1 Answer 1


org-comment-line-break-function unconditionally inserts fill-prefix at the beginning of a new line. The default value of fill-prefix is nil standing for "no prefix". That is not acceptable for insert-before-markers-and-inherit.

Customize the variable fill-prefix to the string "" instead of nil and the error will go away.

The variable fill-prefix is buffer-local. If the global initialization of fill-prefix with the empty string gives you any trouble you can also set it specifically for org files in org-mode-hook. Just copy the following text into your init file.

(defun org-config-fill-prefix ()
  "Set `fill-prefix' to the empty string."
  (setq fill-prefix ""))

(add-hook 'org-mode-hook #'org-config-fill-prefix)
  • 1
    Thanks for your reply but it doesn't change the problem unfortunately. It doesn't yield an error anymore but still not working (it breaks the tag at the end of line)
    – loukios
    Commented Feb 20, 2018 at 15:29
  • 1
    @loukios org-mode is at version 9.1.6 now. Could you update and look whether the problem persists?
    – Tobias
    Commented Feb 20, 2018 at 16:01
  • Shouldn't org-comment-line-break-function be modified so that if fill-prefix is nil it does TRT, rather than passing it on to a function that is expecting strings?
    – NickD
    Commented Mar 31, 2022 at 17:46

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