In org-imenu-get-tree,
          (when (<= level n)
            (looking-at org-complex-heading-regexp)
            (setq head (org-link-display-format
                         (org-match-string-no-properties 4))
               m (org-imenu-new-marker))

should probably be
           (when (and (<= level n)
                    (looking-at org-complex-heading-regexp))
            (setq head (org-link-display-format
                         (org-match-string-no-properties 4))
               m (org-imenu-new-marker))
            ...

i had an headling which (accidentally) consisted of just a todo keyword, making (org-match-string-no-properties 4) nil.