Swift3 swift 3中UICollectionView的自定义布局

我在xcode8中定制了UICollectionView的布局，但是当我试图查看布局时，我的应用程序崩溃了。这是我得到的错误，我不知道为什么会发生这种情况

*** Assertion failure in -[UICollectionView _createPreparedCellForItemAtIndexPath:withLayoutAttributes:applyAttributes:isFocused:notify:], /BuildRoot/Library/Caches/com.apple.xbs/Sources/UIKit_Sim/UIKit-3599.6/UICollectionView.m:2090
 *** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'the collection view's data source did not return a valid cell from -collectionView:cellForItemAtIndexPath: for index path <NSIndexPath: 0x6080004221e0> {length = 2, path = 0 - 0}'

非常感谢您的帮助

    override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {

        return attributesList
    }

    override func layoutAttributesForItem(at indexPath: IndexPath) -> UICollectionViewLayoutAttributes? {

        return attributesList[indexPath.row]
    }