关于NSUserDefaults

在NSUserDefaults中传入一个数组 数组中包括5个字典类型的变量，为什么传不成功？
求大神看看
NSDictionary *dic1 = @{stu1.name : stu1};
NSDictionary *dic2 = @{stu2.name : stu2};
NSDictionary *dic3 = @{t1.name : t1};
NSDictionary *dic4 = @{t2.name : t2}
NSArray *array2 = @[dic1 , dic2 , dic3 , dic4]
NSUserDefaults *ud = [NSUserDefaults standardUserDefaults];
[ud setObject:array2 forKey:@"key-all"]; //这一步失败
[ud synchronize];
NSMutableArray *newarray = [[NSMutableArray alloc]init];
// NSUserDefaults *ud = [NSUserDefaults standardUserDefaults];
newarray = [ud objectForKey:@"key-all"];
NSLog(@"%@" , newarray);