I stumbled upon a bug that makes the PDF generation step explode. I had an aside that did not use the type symbol consistently, something in the style of
(my mistake completely, stupid copy/paste, but took me a good half-hour triaging through the chapters and re-running the preview process to find the offending lines. the preview process didn't really report any useful errors. just mentioning it here as a low priority improvement, perhaps you could detect this in the markdown parsing phase and complain more meaningfully.