Unable to save relabeled words as "O"

[blvns]

I am unable to relabel a (single-word) entity as O -- even when I save the document, it is labeled as a named entity when I return to it. I am able to change the type of entity across saves.

The example I noticed this issue on is Document 32/n01032 in the EN PUD dataset.

Edit: After testing this more, I think the issue is that I can't unlabel the only entity in a document and save it as blank/unlabeled