Recursive Function For Hierarchically Data

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
namespace amief.Models
{
public class WebsiteModels
{
public static void getPagesForPage(int pageId, dbDataContext db, List<page> myPages)
{
var pages = (from p in db.pages
where p.pageParent == pageId
select p);
foreach (var item in pages)
{
myPages.Add(item);
getPagesForPage(item.pageId, db, myPages);
}
}
}
}

calling the procudure

List<page> myPages = null;
WebsiteModels.getPagesForPage(0, db,myPages);

i'm getting an error System.NullReferenceException was unhandled by user code Message=Object reference not set to an instance of an object. on line "myPages.Add(item);" I don't understand the error...