[O] Can't email multipart/alternative anymore

[Peter Davis]

I just tried running org-mime-org-buffer-htmlize, which I have used 
successfully in the past. However, this time, I got an empty
message buffer, and this in the mini-buffer:

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

Any clues? I haven't used this in a couple of months, but it did work at one 
time and, as far as I know, I haven't changed anything
that should impact this.

Thanks!

-pd