I found the question, take an easy example:

    * Title1
    *** Title2
    Test[fn:6]
    * Footnotes
    [fn:6]

If there's a footnote is empty (or just following a space or so), the exporter will just return "org-trim: Wrong type argument: stringp, nil"

I think this is a bug because the exporter should told user "there're some footnotes are empty" instead of an human-unreadable error message.

kuanyui 20140513 0014 GMT+8