我收到错误[__NSArrayI objectForKeyedSubscript：]：无法识别的选择器发送到实例

Question

[self.arrEmail removeAllObjects];
self.arrEmail = [[NSMutableArray alloc]init];
for (NSDictionary *object in [dictionary valueForKey:@"heroes"]) {
    NSString *objectName = object[@"user_email"];
    [self.arrEmail addObject:objectName];
}

我的应用程序在这些代码行崩溃。 Xcode说

[__NSArrayI objectForKeyedSubscript:]: unrecognized selector sent to instance.

有人可以帮忙吗？