FACEBOOK SDK v4返回空电子邮件地址

我正在使用facebook sdk v4，我似乎无法获得电子邮件地址。这是我的代码。我搜索了互联网上的每一个链接，尝试了每一件事。也尝试了不同的脚本，但我仍然得到空电子邮件。这是我的代码。

/* INCLUSION OF LIBRARY FILEs*/
    require_once( 'lib/Facebook/FacebookSession.php');
    require_once( 'lib/Facebook/FacebookRequest.php' );
    require_once( 'lib/Facebook/FacebookResponse.php' );
    require_once( 'lib/Facebook/FacebookSDKException.php' );
    require_once( 'lib/Facebook/FacebookRequestException.php' );
    require_once( 'lib/Facebook/FacebookRedirectLoginHelper.php');
    require_once( 'lib/Facebook/FacebookAuthorizationException.php' );
    require_once( 'lib/Facebook/GraphObject.php' );
    require_once( 'lib/Facebook/GraphUser.php' );
    require_once( 'lib/Facebook/GraphSessionInfo.php' );
    require_once( 'lib/Facebook/Entities/AccessToken.php');
    require_once( 'lib/Facebook/HttpClients/FacebookCurl.php' );
    require_once( 'lib/Facebook/HttpClients/FacebookHttpable.php');
    require_once( 'lib/Facebook/HttpClients/FacebookCurlHttpClient.php');
/* USE NAMESPACES */
    use Facebook'FacebookSession;
    use Facebook'FacebookRedirectLoginHelper;
    use Facebook'FacebookRequest;
    use Facebook'FacebookResponse;
    use Facebook'FacebookSDKException;
    use Facebook'FacebookRequestException;
    use Facebook'FacebookAuthorizationException;
    use Facebook'GraphObject;
    use Facebook'GraphUser;
    use Facebook'GraphSessionInfo;
    use Facebook'FacebookHttpable;
    use Facebook'FacebookCurlHttpClient;
    use Facebook'FacebookCurl;
/*PROCESS*/
    //1.Stat Session
     session_start();
    //check if users wants to logout
     if($_REQUEST['logout']==1){
        unset($_SESSION['fb_token']);
     }
    //2.Use app id,secret and redirect url 
    $app_id = 'xxxxxxxxxx';
    $app_secret = 'xxxxxxxxxxxxxxx';
    $redirect_url='http://someurl.com/';
    //3.Initialize application, create helper object and get fb sess
     FacebookSession::setDefaultApplication($app_id,$app_secret);
     $helper = new FacebookRedirectLoginHelper($redirect_url);
     $sess = $helper->getSessionFromRedirect();
    //check if facebook session exists
    if(isset($_SESSION['fb_token'])){
        $sess = new FacebookSession($_SESSION['fb_token']);
    }
    //logout
    $logout = 'http://dryvr.co/fb/?logout=1';
    //4. if fb sess exists echo name 
        if(isset($sess)){
            //store the token in the php session
            $_SESSION['fb_token']=$sess->getToken();
            //create request object,execute and capture response
            $request = new FacebookRequest($sess,'GET','/me');
            // from response get graph object
            $response = $request->execute();
            $graph = $response->getGraphObject(GraphUser::classname());
            // use graph object methods to get user details
            $name = $graph->getName();
            $id = $graph->getId();
            $image = 'https://graph.facebook.com/'.$id.'/picture?width=300';
            $email = $graph->getProperty('email');
            echo "hi $name <br>";
            echo "your email is $email <br><Br>";
            echo "<img src='$image' /><br><br>";
            echo "<a href='".$logout."'><button>Logout</button></a>";
        }else{
            //else echo login
            echo '<a href="'.$helper->getLoginUrl(array('email')).'" >Login with facebook</a>';
        }

我已经使用了$email = $graph->getProperty('email');，但我仍然无法访问电子邮件地址，它返回空白电子邮件。我该怎么办?想法吗?