NSUserDefaults数据保存报错：Attempt to set a non-property-list object.

@interface PaymentModel : NSObject<NSCoding>

//标题
@property (nonatomic, copy)NSString *title;
//图片
@property (nonatomic, copy)NSString *picture;

@end

#import "PaymentModel.h"

@implementation PaymentModel

- (void)encodeWithCoder:(NSCoder *)aCoder {
    [aCoder encodeObject:self.title forKey:@"title"];
    [aCoder encodeObject:self.picture forKey:@"picture"];
}

- (id)initWithCoder:(NSCoder *)aDecoder {
    if (self = [super init]) {
        self.title = [aDecoder decodeObjectForKey:@"title"];
        self.picture = [aDecoder decodeObjectForKey:@"picture"];
    }
    return self;
}

@end