用facebook登录恢复电子邮件

Question

我有这个脚本，它的效果非常好。我遇到的唯一问题是我无法恢复电子邮件地址，它是空的：

FacebookSession::setDefaultApplication( 'appid','secret' );
// login helper with redirect_uri
$helper = new FacebookRedirectLoginHelper('URL');
try {
$session = $helper->getSessionFromRedirect();
} catch( FacebookRequestException $ex ) {
// When Facebook returns an error
} catch( Exception $ex ) {
// When validation fails or other local issues
}
// see if we have a session
if ( isset( $session ) ) {
// graph api request for user data
$request = new FacebookRequest( $session, 'GET', '/me' );
$response = $request->execute();
// get response
$graphObject = $response->getGraphObject();
$fbid = $graphObject->getProperty('id');              // To Get Facebook ID
$fbfullname = $graphObject->getProperty('name'); // To Get Facebook full name
$femail = $graphObject->getProperty('email');    // To Get Facebook email ID
/* ---- Session Variables -----*/
$_SESSION['FBID'] = $fbid;
$_SESSION['FULLNAME'] = $fbfullname;
$_SESSION['EMAIL'] =  $femail;
 header("Location: index.php");
} else {
$loginUrl = $helper->getLoginUrl();
header("Location: ".$loginUrl);
}

我正确获得$ fbid和$ fbfullname，但我的$ femail是空的。知道如何解决这个问题吗？