Objective-C中未声明属性错误

我想知道这是怎么回事?

typedef enum {
    N4LoupeTypeRound,
    N4LoupeTypeRectangle,
} N4LoupeType;

@interface N4LoupeLayer : CALayer {
    N4LoupeType _type;
    UIView *_originalView;
    CALayer *_mask;
    CALayer *_overlay;
}

@property (nonatomic) N4LoupeType type;
@property (nonatomic, assign) UIView *originalView;

@end

.m文件:

#import "N4LoupeLayer.h"

@interface N4LoupeLayer (Privates)

@property (nonatomic, retain) CALayer *mask;
@property (nonatomic, retain) CALayer *overlay;

@end

@implementation N4LoupeLayer

@synthesize type = _type;
@synthesize originalView = _originalView;
@synthesize mask = _mask;
@synthesize overlay = _overlay; // ******I GET THE ERROR HERE********* 

@end