[O] org-mime-org-buffer-htmlize can't find back-end

[Peter Davis]

I used to be able to use org-buffer-org-mime-htmlize to send my org documents 
as multipart/alternative html & text email. For some
reason, this is stopped working, and now displays

Unknown "nil" back-end: Aborting export

In the *Messages* buffer, I see

org-export-barf-if-invalid-backend: Unknown "nil" back-end: Aborting export

So how is the back-end supposed to be defined? (And why might it have broken?) 
I haven't not made any related changes in a *long*
time.


Thank you.

-pd