iOS使用sortDescriptor设置为localIdentifier来获取集合

Question

似乎我无法获得由localIdentifier排序的获取结果。这是代码：

let sortDescriptor = NSSortDescriptor(key: "localIdentifier", ascending: true)
let options = PHFetchOptions()
options.sortDescriptors = [sortDescriptor]
let albums = PHCollection.fetchTopLevelUserCollections(with: options)
print(albums.count)

我有这个错误：

Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'Unsupported sort descriptor in fetch options: (localIdentifier, ascending, compare:)'

但是在Apple doc中，localIdentifier可以用作密钥。