You cannot create new posts on a blog site that uses a customized PostCategory content-type in Windows SharePoint Services 3.0

using (SPSite site = new SPSite("http://localhost/blog"))  //for the Url of the blog web
{
   SPWeb web = site.OpenWeb();
   Console.WriteLine(web.Title);
   SPList postList = web.Lists["Posts"];
   SPList categoryList = web.Lists["Categories"];
   SPFieldLookup postcategoryLookup = (SPFieldLookup)postList.Fields.GetField("PostCategory");
   String lookupList = categoryList.ID.ToString("B").ToUpper();
   postcategoryLookup.LookupList = lookupList;  //this line will throw exception if the List is already set
   postcategoryLookup.LookupField = "Title";
   postcategoryLookup.Update();
   Console.WriteLine(postcategoryLookup.SchemaXml);
}