org-comment-line-break-function does not handle fill-prefix being set to
nil, which is the default value for fill-prefix. This means that pressing
M-j inside an org-mode buffer in a vanilla installation of Emacs results in
an error. From looking at other callers of
insert-before-markers-and-inherit it is clear that a guard against
fill-prefix being nil is missing.

This has been reported beforehttps://
list.orgmode.org/87o861o9sh.fsf@aquinas.i-did-not-set--mail-host-address--so-tickle-me/

Cheers,
Javier Olaechea

-- 
"I object to doing things that computers can do." — Olin Shivers