I take it you're OK now. The error was probably "list index out of range"
For those multipart stories that are not grouped as a series I find the easiest way is to download all the parts as html using the fanfiction plugin. I have created a different library for them to make them easy to find.
Double clicking on the book cover will open the zip file. I drag all the html files to a temporary folder one by one and then select the lot and drag them into Sigil (the latest version allows drag and drop), run a few regex search and replace to massage the book into something useable.
I copy the metadata into notebook++, add the metadata tags around it, copy it into the content.opf file and then save and drag into Calibre .If everything is OK I delete the individual downloads, add cover etc and move to main library.