__NSCFString objectForKeyedSubscript: exception

I take datas from server. My app work fine in Sinulator and test device iPhone 4s, but one man have problem on iPod 4. He get exception:

-[__NSCFString objectForKeyedSubscript:]: unrecognized selector sent to instance 0x1d263a20

I cann't use this device so I write code to know where crash was.

 if (![dictionaryRest[@"compliments"] isEqual:[NSNull null]]) {

       NSMutableArray *array = [NSMutableArray new];

       NSMutableArray *firstArray = [NSMutableArray new];

       for (NSDictionary *dic in dictionaryRest[@"compliments"]) {

            Compliment *compl = [Compliment new];

            if (![dic[@"ID_promotions"] isEqual:[NSNull null]])

                compl.ID = [dic[@"ID_promotions"] integerValue];

So in last 2 strings this exception was. What the reason of this? So I understand that I need use

if ([dict objectForKey:[@"compliments"])

instead

if (![dict[@"compliments"] isEqual:[NSNull null]])

and in all another cases.

I test now and I have in my dictionary for ID: