给nsdictionary的key添加一个数组

想给nsdictionary的key添加一个数组,其中每个key都是一个question，每个question对应多个answer：

Question1 = [Answer1], [Answer2] , [Answer3];

Question2 = [Answer1], [Answer2] , [Answer3];

然后我尝试用下面的代码时报错：

`
No Visible @interface for NSDictionary declares the selector 'setObject::forKey'
`
代码：

//answers
_surveyAnswers1 = [[NSMutableDictionary alloc] init];
NSArray *myArrayAnswers1 = [NSArray arrayWithObjects:@"One",@"Two", nil];
NSArray *myArrayAnswers2 = [NSArray arrayWithObjects:@"Three",@"Four", nil];
[_surveyAnswers1 setObject:myArrayAnswers1 forKey:@"FirstKey"];
[_surveyAnswers1 setObject:myArrayAnswers2 forKey:@"SecondKey"];

不知道怎么正确添加？