Comment 2 for bug 101413

OK, changed the xmlexport of tocs to assume unlimited when there is no toc_depth attribute. Unlimited is the default in the widgets-based renderer, as well as AutoTOC content type.

However, if you do the steps to replicate, and instead switch back to kupu, you get an error:
  Module Products.SilvaDocument.transform.kupu.silva, line 459, in convert
ValueError: invalid literal for int(): None