wrong-type-argument char-or-string-p nil when doing RET in org-mode

Question

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)
insert-before-markers-and-inherit(nil)
org-comment-line-break-function(t)
default-indent-new-line(t)
do-auto-fill()
self-insert-command(1)
newline()
org-return()
funcall-interactively(org-return)
call-interactively(org-return nil nil)
command-execute(org-return)

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.

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)