ios - 无法识别的选择器（NSMutableArray+NSString）

Question

一个非常基本但令人费解的错误；

NSString *selectedCategory = @"Rock";

if (self.quoteOpt.selectedSegmentIndex == 1) 
{
        selectedCategory = @"Programming";
}

//filter array by ti tle using predicate
NSPredicate *predicate = [NSPredicate predicateWithFormat:@"Genre == %@", selectedCategory];
NSArray *filteredArray = [self.movieQuotes filteredArrayUsingPredicate:predicate];

int array_tot = [filteredArray count];

if(array_tot > 0)
{
   int index = (arc4random() % array_tot);
   NSMutableArray *a = [filteredArray objectAtIndex:index];
   NSString *s = [a objectAtIndex:1];
   self.quoteText.text = [NSString stringWithFormat:@"Genre \n\n %@",s];
}
else
{
   self.quoteText.text = [NSString stringWithFormat:@"No quotes to display."];
}

我对它的行为方式感到困惑，产生的错误来自

NSString *s = [a objectAtIndex:1];

我做了一个 po hexaddress，它似乎是 int 但它是从哪里来的？

po'ing *a 和 *s 结果分别为；

*一个：

(NSMutableArray *) $3 = 0x088941f0 {
  Genre = Rock;
  Name = Creed;
  "New item" = Higher;
 }

*s:

(int) $4 = 143213040 {
  Genre = Rock;
  Name = Creed;
  "New item" = Higher;
}

我想不通，我一直在寻找，但该错误本质上非常笼统/模糊，无法获得具体答案。（我的两分钱）

TIA