I get this warning even though the key doesn't actually exist
If I use a different key it works fine, but I don't know why this key won't work
TL;DR
Developers are experiencing a warning for an attribute key that doesn't exist. Deleting and recreating the collection seems to solve the issue. It's possible that when the attribute was deleted, it wasn't completely removed. Using a different key works fine, but the reason for the specific key not working is unclear.