获取到相册的所有照片并展示在页面上，为什么获取到的系统照片值为空？

let fetchOption: photoAccessHelper.FetchOptions = { 
  fetchColumns: ['size'], 
  predicates: predicates 
}; 
let fetchResult: photoAccessHelper.FetchResult<photoAccessHelper.PhotoAsset> = await this.phAccessHelper.getAssets(fetchOption); 
let photoAsset: photoAccessHelper.PhotoAsset = await fetchResult.getFirstObject(); 
let photoAssetSize: photoAccessHelper.MemberType = photoAsset.get('size');