I suspect that what's happening is that I'm identifying the first word in the new page, which isn't a number. So I'm back to my previous method.

Update: I changed the targets to the empty first example and the plugin ran perfectly. However, using the APNX output in Calibre to send the book to my Kindle still doesn't produce accurate page numbers. They're clearly not using the PageList for anything. Maybe KindleCreate does. (I can't find a KindleGen, so KindleCreate is probably the latest version of that.)