MongoDB server 4.0.1 sends 'drop' event in change stream.
But OperationType class can't handle 'drop' event, and throws IllegalArgumentException.
It should support 'drop' operation type. And I think it should support 'unknown' type for fail safe purpose.
And when the exception occurs in ChangeStreamBatchCursor.convertResults(), other events following 'drop' event (like as 'invalidate') is not handled(dropped), so they are not notified to the application.