Why I cant retrieve the facebook gender? [Laravel 5.1 Socialite]

Hi guys I need a little help here, I just made an auth controller with Laravel Socialite. First time I just retrieve the name and email data, and then I try to to retrieve the gender but it comes with an error that says

Undefined property: Laravel\Socialite\Two\User::$gender

Someone please help me to solve my problem, oh yeah my web still in development mode but I think it's not a problem because in facebook developer documentation said I still have access to basic information like first name, last name, profile picture, gender and age range.

Controller Source Code:

public function redirectToProvider()
{
    return Socialite::driver('facebook')->redirect();
}


public function handleProviderCallback()
{
    try {
        $user = Socialite::driver('facebook')->user();
    } catch (Exception $e) {
        return Redirect::to('login/facebook');
    }

    $authUser = $this->findOrCreateUser($user);

    Auth::login($authUser, true);

    return Redirect::to('home');
}


private function findOrCreateUser($fbUser)
{
    if ($authUser = User::where('email', $fbUser->email)->first()) {
        return $authUser;
    }
    $role = "member";
    return User::create([
        'firstname' => $fbUser->name,
        'email' => $fbUser->email,
        'gender' => $fbUser->gender,
        'role' => $role
    ]);
} 

via Chebli Mohamed