IOS- “This class is not key value coding-compliant”

在ios应用中声明：

@interface UserRegistrationRequest : BaseServiceRequest
@property (nonatomic, retain) NSString *username;
@property (nonatomic, retain) NSString *password;
@property (nonatomic, retain) NSString *secretQuestionID;
@property (nonatomic, retain) NSString *secretQuestionAnswer;
@property (nonatomic, retain) UserProfile *profile;

@end

@interface UserProfile : NSObject
@property (nonatomic, retain) NSString *emailAddress;
@property (nonatomic, retain) NSString *firstName;
@property (nonatomic, retain) NSString *lastName;
@property (nonatomic, retain) NSData *profileImage;
@property (nonatomic, retain) NSNumber *dateOfBirth;
@end

我认为这些类应该是键值编码兼容
运行下面的代码：

NSString *propKey = @"profile.firstName";
NSString *propVal = [self.registrationRequest valueForKey:propKey];

报出异常：

[<UserRegistrationRequest 0xb6187f0> valueForUndefinedKey:]: this class is not key value coding-compliant for the key profile.firstName.

不知道应该怎么解决？