Hello,

I believe a regression was introduced in
https://code.orgmode.org/bzg/org-mode/commit/d81a1d088c74e605c99e90a2835c55df5144f43e

I am unable to come with a minimal example, but I can link to the actual
Org file that causes the failure. If I try to move that subtree along with
the "included" subtrees into a separate Org file, the export works fine.

- Download
https://raw.githubusercontent.com/kaushalmodi/ox-hugo/master/test/site/content-org/all-posts.org
(you can wget or curl that file using that URL).
- Ensure that point is in the "* Alert Shortcode Lookalike" heading (around
line 3416).
- Do C-c C-e C-s h h

You should get an error like this:

Debugger entered--Lisp error: (wrong-type-argument stringp nil)
  looking-at(nil)
  org-element-context()

org-export--prepare-file-contents("/home/kmodi/stow/pub_dotfiles/emacs/dot-emacs.d/elisp/ox-hugo/test/site/content-org/
all-posts.org" "3311-3358" 0 5 1 #<hash-table equal 0/65 0x15ea311>
"/home/kmodi/stow/pub_dotfiles/emacs/dot-emacs.d/elisp/ox-hugo/test/site/content-org/
all-posts.org")
  org-export-expand-include-keyword()
  org-export-as(html t nil nil (:output-file
"alert-short-code-lookalike.html"))
  org-export-to-file(html "alert-short-code-lookalike.html" nil t nil nil
nil)
  org-html-export-to-html(nil t nil nil)
  org-export-dispatch(nil)
  funcall-interactively(org-export-dispatch nil)
  call-interactively(org-export-dispatch nil nil)
  command-execute(org-export-dispatch)

Let me know if this error can be reproduced by the above steps
-- 

Kaushal Modi