NSMutableArray removeObjectAtIndex：抛出无效的参数异常

I'm writing an application to show some news from a portal. The news are fetched using a JSON file from the Internet and then stored into a NSMutableArray using the CoreData Model. Obviously a user can't delete the news from the JSON file on the Internet, but he can hide them locally. The problems come out here, where I have the following code:I'm writing an application to show some news fr