NSUserDefaults setObject:forKey:尝试插入非属性列表对象

我想我在这里做了一些非常明显的错误，但我想不通！我正在尝试将包含NSString、NSNumbers和NSBoolean的NSMutableDictionary保存到iOS 7.1上的NSUserDefaults中。

以下是字典定义：

- (NSMutableDictionary *)hotLevelsDict
{
    if ( (!_hotLevelsDict) || ([_hotLevelsDict count] < 1) )
    {
        _hotLevelsDict = [NSMutableDictionary dictionaryWithDictionary:
                          @{@100: [NSMutableDictionary dictionaryWithDictionary:@{@"time": @20, @"points": @50, @"complete": @NO}],
                            @200: [NSMutableDictionary dictionaryWithDictionary:@{@"time": @20, @"points": @100, @"complete": @NO}],
                            @500: [NSMutableDictionary dictionaryWithDictionary:@{@"time": @20, @"points": @200, @"complete": @NO}],
                            @1000: [NSMutableDictionary dictionaryWithDictionary:@{@"time": @20, @"points": @200, @"complete": @NO}],
                            @2000: [NSMutableDictionary dictionaryWithDictionary:@{@"time": @20, @"points": @500, @"complete": @NO}],
                            @5000: [NSMutableDictionary dictionaryWithDictionary:@{@"time": @20, @"points": @1000, @"complete": @NO}],
                            @10000: [NSMutableDictionary dictionaryWithDictionary:@{@"time": @20, @"points": @1000, @"complete": @NO}],
                            @20000: [NSMutableDictionary dictionaryWithDictionary:@{@"time": @10, @"points": @1000, @"complete": @NO}],
                            @50000: [NSMutableDictionary dictionaryWithDictionary:@{@"time": @20, @"points": @1000, @"complete": @NO}],
                            @100000: [NSMutableDictionary dictionaryWithDictionary:@{@"time": @20, @"points": @1000, @"complete": @NO}],
                            @200000: [NSMutableDictionary dictionaryWithDictionary:@{@"time": @20, @"points": @1000, @"complete": @NO}],
                            @300000: [NSMutableDictionary dictionaryWithDictionary:@{@"time": @20, @"points": @1000, @"complete": @NO}],
                            @400000: [NSMutableDictionary dictionaryWithDictionary:@{@"time": @20, @"points": @1000, @"complete": @NO}],
                            @500000: [NSMutableDictionary dictionaryWithDictionary:@{@"time": @20, @"points": @1000, @"complete": @NO}],
                            @600000: [NSMutableDictionary dictionaryWithDictionary:@{@"time": @20, @"points": @1000, @"complete": @NO}],
                            @700000: [NSMutableDictionary dictionaryWithDictionary:@{@"time": @20, @"points": @1000, @"complete": @NO}],
                            @800000: [NSMutableDictionary dictionaryWithDictionary:@{@"time": @20, @"points": @1000, @"complete": @NO}],
                            @900000: [NSMutableDictionary dictionaryWithDictionary:@{@"time": @5, @"points": @50000, @"complete": @NO}],
                            @950000: [NSMutableDictionary dictionaryWithDictionary:@{@"time": @2, @"points": @50000, @"complete": @NO}]
                            }];
    }
    return _hotLevelsDict;
}

保存它：

[defaults setObject:self.hotLevelsDict forKey:@"hotLevelsDict"];

我已经尝试打印字典中所有子对象的类，它们都是标准的，而且我正在对以非常相似的方式构建的另一个字典做同样的事情，它工作得很好。

有人能指出我的错误吗？

谢谢！