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

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