I've checked with four books: three created with calibre and a purchased one (no DRM). The books created by calibre give the same error: string index out of range (and calibre plugin fails too). The purchased book give an error about a bad reference in opf.
None of them are validated by EpubCheck, so if it's necessary, I think I'll pass.
The log at the plugin is:
If you want one of the epubs, I can send it to you.