objective-c 局部变量报错?报错

这是Deck.h的代码

#ifndef Machismo_Deck_h

#define Machismo_Deck_h

#import <Foundation/Foundation.h>

#import "Card.h"

@interface Deck : NSObject

-(void)addCard:(Card *)card atTop:(BOOL)atTop;

-(void)addCard:(Card *)card;

-(Card *)drawRandomCard;

@end

#endif

下面是Deck.m的代码

#import "Deck.h"

@interface Deck()

//private methods are declared here

@property (strong, nonatomic) NSMutableArray *cards;

@end

@implementation Deck

-(NSMutableArray *)cards{

    if (!_cards) {

        _cards = [[NSMutableArray alloc] init];

    }

    return _cards;

}

-(void)addCard:(Card *)card atTop:(BOOL)atTop{

    if (atTop) {

        [self.cards insertObject:card atIndex:0];

    } else {

        [self.cards addObject:card];

    }

}

-(void)addCard:(Card *)card{

    [self addCard: card atTop: NO];

}

-(Card *)drawRandomCard{

    Card *randomCard = nil;//Parse Issue Expected expression

    if ([self.cards count]) {

        unsigned index = arc4random() % [self.cards count];

        randomCard = self.cards[index];//Semantic Issue Use of undeclared identifier 'randomCard'

        [self.cards removeObjectAtIndex:index];

    }

    return randomCard;//Semantic Issue Use of undeclared identifier 'randomCard'

}

@end

红色的是错误的地方，刚接触Objective-C～ 请老司机看看为什么报错～