EXC_BAD_ACCESS while trying to observe NSOperation properties

[wiistriker]

    NSBlockOperation *operation = [NSBlockOperation blockOperationWithBlock:^{
        NSLog(@"some task");
    }];

    [operation addObserver:self
                forKeyPath:@"isExecuting"
                   options:NSKeyValueObservingOptionNew
                   context:nil
                 withBlock:^(NSDictionary *change, void *context) {
                     BOOL isExecuting = [change[NSKeyValueChangeNewKey] boolValue];
                     if (isExecuting) {
                         NSLog(@"operation is executing");
                     }
                 }];

    [operation start];

throw EXC_BAD_ACCESS at https://github.com/sleroux/KVO-Blocks/blob/0.1.0/KVO%20Blocks/NSObject%2BKVOBlocks.m#L30