Update collection with dot notation not working

I’m trying to update a collection like so…

          _id: groupId
          $set: {
            "layout.boxes": data.layoutBoxes,

However, since upgrading to 1.8, I’m now seeing this error:

Error: key layout.boxes must not contain '.'

This appears to still be valid syntax in Mongo 4.0: https://docs.mongodb.com/manual/reference/operator/update/set/

Do I need to switch syntax, or is this a bug?


Never mind. I’ve just realised that the problem is with a parent collection that this one inherits from.