I've gotten the attached backtrace a couple of times in the past few
days. Also same error with org-cache-sync. I'm not sure how reproducible
it is and I have to admit that I've been running with a possibly tainted
emacs/org for the last few days. I'll restart emacs and try to keep it
pristine and see if it happens again, but I thought I'd post the
backtrace just in case it is real.