Wordpress get_avatar() not working

I try to get avatar display from wordpress user. But it can not work. Well... I'm stuck here for 2 days to solve this so I running here.. Here is my code:

public function authUser(ConnectionInterface $conn, array $cookies) {
    global $current_user;
    $current_user = null;
    $wpDir = $conn->Config['wordpress']['dir'];

    $this->debug("WP Autorization cookies:"); $this->debug($cookies);

    foreach ($cookies as $key => $value) {
        $_COOKIE[$key] = urldecode($value);
    }
    require_once($wpDir . '/wp-config.php');
    require_once($wpDir . '/wp-includes/wp-db.php');
    require_once($wpDir . '/wp-includes/pluggable.php');
    $userInfo  = wp_get_current_user();
    if(!$userInfo || !$userInfo->ID){
        return null;
    }

    //get avatar from Wordpress
    $avatar = get_avatar( $userInfo->ID, 50 );
    return array(
        'id'            => $userInfo->ID,
        'name'          => $userInfo->display_name,
        'image'         => $avatar
    );
}

$userInfo->ID and $userInfo->display_name is working good. I got the ID and display name. But the avatar is not working. Any help and suggestion really appreciated. Thank you.