NSOperation和ARC中NSManagedObjectContext保存方法上的EXC_BAD_ACCESS,为什么?

在打开ARC的NSOperation中保存NSManagedObjectContext时,我发现了一些问题。没有ARC之前一切都很好。保存时总是给出EXC_BAD_ACCESS。代码看起来像这样:

//on the main thread
-(void)someFunc
{
    array = ... //fetching an array of entities from a core data
    for(SomeEntity * obj in array)
    {
         NSSomeOperation * op = [[NSSomeOperation alloc] initWithValue:[obj someField]];
         //start an operation
    }
}

//NSSomeOperation implementation
//...
- (void)main {
    //some code
    NSError * error = nil;
    [mainContext lock];
    if (![mainContext save:&error]) {    //<--- HERE EXC_BAD_ACCESS
       //process error
    }      
    [mainContext unlock];
    //some code
}
//...

使用[mainContext setRetainsRegisterdObjects:YES]和objectWithID并不能解决此问题。

EXC_BAD_ACCESS(代码=1)
EXC_BAD_ACCESS(代码=13)

-[__NSCFType contextDidSave:]: unrecognized selector sent to instance 0x7fc5c505d940

An observer of NSManagedObjectContextDidSaveNotification illegally threw an exception.

Objects saved = {
    inserted = "{(\n)}";
    updated = "{(\n    <SomeEntity: 0x7fc5c55b6220> (entity: SomeEntity; id: 0x7fc5c5052b20 ... )}"; } 
and exception = -[__NSCFType contextDidSave:]: unrecognized selector sent to instance 0x7fc5c505d940 with userInfo = (null)

我使用单独的托管对象上下文,并在此NSOperation中获取我的托管对象。

也许这与核心数据错误或ARC有关?也许ARC会清理一些必须保存的对象? 因为,没有ARC,一切都很好,一切都正常。当我打开ARC-EXC_BAD_ACCESS时。

有人知道为什么会发生吗?