Swift 3核心数据 – NSExpression forFunction：“sum：”抛出错

我有日子和任务.一天有很多任务.每个任务都有一个名为“点”的属性,我想总结当天任务的所有点.我使用下面的代码(在教程中的Core Data,swift 2版本中找到)并尝试修改它为swift 3(我还添加了一个谓词,但这并不重要).但是当我运行此代码时,我收到此错误：

Could not cast value of type ‘NSKnownKeysDictionary1’ (0x10d02d328) to ‘MyProject.Day’

我究竟做错了什么？

// sum current day's task points 
    let sumRequest: NSFetchRequest<Day> = Day.fetchRequest()
    sumRequest.resultType = .dictionaryResultType

    sumRequest.predicate = NSPredicate(format: "SELF == %@",argumentArray: [Project.Days.current])

    let expressionDescription = NSExpressionDescription()
    expressionDescription.name = "sumOfPoints"
    expressionDescription.expression = NSExpression(forFunction: "sum:",arguments: [NSExpression(forKeyPath: "tasks.points")])
    expressionDescription.expressionResultType = .integer16AttributeType

    sumRequest.propertiesToFetch = [expressionDescription]

    do {
        let results = try managedObject.fetch(sumRequest) as! [NSDictionary] // Here's the line that causes the error
        print("DEN:",results)
    } catch let error as NSError {
        print("DEN:",error.localizedDescription)
    }

我认为这与这一行有关：

let sumRequest: NSFetchRequest<Day> = Day.fetchRequest()

在这里,我明确地说结果将是一天(我相信这是一个新的快速3？).但我不知道如何改变这一点.

谢谢！

let sumRequest: NSFetchRequest<NSFetchRequestResult> = Day.fetchRequest()