Upsert nil in place of current value

I'm running an upsert that replaces the current value (e.g. a non-nil value) with a nil, but the nil fails to get upserted. Upserts of any other value seems to work. Is this unexpected behavior? Am I missing something?

Start by reproducing the reported upsert that replaces a non-nil value with nil, comparing it with upserts of other values. The issue is resolved when the nil value is successfully persisted and the behavior is covered by a regression test.

Written by the indexing model from the issue text.