关于随机分配4个图片到iPhone按钮中 的问题

代码：

-(void)RandomButtonImage {

    NSMutableArray *images = [NSMutableArray arrayWithObjects:@"Character_1_1.png",@"Character_1_2.png",@"Character_2_1.png",@"Character_2_2.png" nil];

    NSArray *Buttons = [NSArray arrayWithObjects:AnsButton1,AnsButton2,AnsButton3,AnsButton4, nil];

    for (UIButton *btn in Buttons) {
        int randomIndex = random() % images.count;
        UIImage *img = [images objectAtIndex:randomIndex];
        [btn setImage:img forState:UIControlStateNormal];
        [images removeObjectsAtIndexes:randomIndex];
    }
}

但是运行这段代码之后最后一行报错:
`
[imagesremoveObjectsAtIndexes:randomIndex];
Implicit Conversion of 'int' to 'NSindexset' is disallowed with ARC
`