Most of our options structs include the serde_with::skip_serializing_none attribute to avoid sending null values to the server for unspecified options. We don't include this attribute on DropCollectionOptions, which means that if a user specifies None for write_concern, the server will return a TypeMismatch error. It's likely that this hasn't been encountered since write_concern is the only field in DropCollectionOptions, so users would likely just omit the options completely rather than specifying None; however, we should still fix this to avoid unnecessary errors.