Weird NSZombie error in nsmutuablearray

Question

I had faced a weird error for a few days ago. Luckly, i had solved this error about in 4-5 day. However, i just wonder that the error why happen.

First of all, i will describe the situation with same sample of code.

@interface SampleViewController ()

@property (nonatomic, strong) NSMutableArray *selectedIssue;
@property (nonatomic, strong) NSOperationQueue *queueJSON;
@property (nonatomic, strong) NSOperationQueue *queueImage;

@end

@implementation SampleViewController

/** blah blah blah*/

- (void) fillIssueArrayMethodWithIssueId:(NSInteger) selectedId {

    NSString *requestURL = [NSString stringWithFormat:@"%@Issues/Get/?id=%d", kAPIRootURL, selectedId];

    NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:[NSURL URLWithString:requestURL] 
                                                           cachePolicy:NSURLRequestReloadIgnoringLocalCacheData 
                                                       timeoutInterval:kNetworkTimeOut];

    AFHTTPRequestOperation *op = [[AFHTTPRequestOperation alloc] initWithRequest:request];
    op.responseSerializer = [AFJSONResponseSerializer serializer];

    [op setCompletionBlockWithSuccess:^(AFHTTPRequestOperation *operation, id response) {
        NSData *JSONData = [NSData dataWithContentsOfFile:filePath
                                                  options:NSDataReadingMappedIfSafe 
                                                    error:nil];
        NSMutableArray *responseObject = [NSJSONSerialization JSONObjectWithData:JSONData 
                                                                         options:NSJSONReadingMutableContainers 
                                                                           error:nil];

        if(responseObject) {
            if([responseObject isKindOfClass:[NSArray class]]) {
                _selectedIssue = [NSMutableArray arrayWithArray:responseObject];
            }
        }

    } failure:^(AFHTTPRequestOperation *operation, NSError *error) { }];

    [_queueJSON addOperation:op];
}

-(void)startDownloadImages {

    NSArray *objPaths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
    NSString *strPath = [objPaths lastObject];

    if(!_queueImage)
        _queueImage = [[NSOperationQueue alloc] init];

    _queueImage.maxConcurrentOperationCount = 3;

    NSBlockOperation *completionOperation = [NSBlockOperation new];

    for (__block NSDictionary *object in _selectedIssue) {
            // Photos
            NSString *imgURL = object[@"ImageUrl"];
            NSString *strImageFile = [NSString stringWithFormat:@"%@_%@", object[@"Id"], [imgURL lastPathComponent]];
            __block NSString *strImagePath = [NSString stringWithFormat:@"%@/Images/Issues/%@", strPath, strImageFile];

            NSURLRequest *request4Images = [NSURLRequest requestWithURL:[NSURL URLWithString:imgURL]];

            AFHTTPRequestOperation *operation4Images = [[AFHTTPRequestOperation alloc] initWithRequest:request4Images];

            [operation4Images setCompletionBlockWithSuccess:^(AFHTTPRequestOperation *operation, id rawResult) {

                 BOOL isImageWrited = [rawResult writeToFile:strImagePath options:NSDataWritingAtomic error:nil];

                 if(isImageWrited) {
                     NSLog(@"Image Write Success : %@", operation.request.URL);
                 } else {
                    NSLog(@"Image Write Error: %@", imageWriteError.description);
                 }

            } failure:^(AFHTTPRequestOperation *operation, NSError *error) {
                NSLog(@"Image Write Failed : %@ : %ld", operation.request.URL, (long)operation.response.statusCode);
            }];

            [completionOperation addDependency:operation4Images];
    }

    [_queueImage addOperations:completionOperation.dependencies waitUntilFinished:NO];
    [_queueImage addOperation:completionOperation];
}

@end

Here is problem;

In application lifecycle, fillIssueArrayMethodWithIssueId method get JSON data to mutuablearray first. Then start to download images by getting image URL from the array.

Each second time, i want to access the _selectedIssue in fillIssueArrayMethodWithIssueId method, my app got crash (SIGSEGV)

In according to long investigation, _selectedUssue become instantly zombie object when StartDownloadImages method finished. However in this method i have never reallocated this array. I have just read values in this array. I am sure that the array become zombie object in this method so the issue fixed when removed "__block" in for loop.

So the question, __block type how could affect the _selectedIssue arrays object ?

I just want to know what i am missing...

By the way,i have been tried in StartDownloadImages method before the for loop, i had create temporary array and this temp array just initiated with _selectedIssue mutableCopy or just copy. However, i again faced with zombie problem.