无法转换泛型类型

Question

为什么我会收到此错误？

  

无法转换FetchResultsController的类型＆lt;实体＆gt;预期   FetchedResultsController的参数类型＆lt; NSManagedObject＆gt;

protocol FetchedResultsControllerDelegate: class {
  func fetchedResultsControllerWillChangeContent(_ controller: FetchedResultsController<NSManagedObject>)
}

open class FetchedResultsController<Entity: NSManagedObject>: NSObject {

  weak var delegate: FetchedResultsControllerDelegate?

  func controllerWillChangeContent(_ controller: NSFetchedResultsController<NSFetchRequestResult>) {
    delegate?.fetchedResultsControllerWillChangeContent(self) // here
  }

}