I am seeing this error from org-mode when trying to export to HTML. The
main org file has an include line like so:

#+INCLUDE: "sisyphus.org" :lines "28-"

The error occurs on a clean emacs -q run from head of Emacs-25.

I don't quite understand where the error is coming from, given that this
simple export has nothing to do with agenda files.

Org-files are attached.

Thanks!

Phil

Debugger entered--Lisp error: (wrong-type-argument stringp nil)
  file-exists-p(nil)
  org-check-agenda-file(nil)
  org-agenda-prepare-buffers((nil))
  org-map-entries(#[nil "\300\301 !\207" [org-reduced-level org-current-level] 2])
  org-export--prepare-file-contents("/home/phillord/emacs/sisyphus/sisyphus.org" "28-" 0 1)
  org-export-expand-include-keyword()
  org-export-as(html nil nil nil (:output-file "./sisyphus-doc.html"))
  org-export-to-file(html "./sisyphus-doc.html" nil nil nil nil nil)
  org-html-export-to-html(nil nil nil nil)
  org-export-dispatch(nil)
  funcall-interactively(org-export-dispatch nil)
  call-interactively(org-export-dispatch nil nil)
  command-execute(org-export-dispatch)