template T4: "type xdocument not defined"

I got this error when i execute my template T4: "type xdocument not defined" !!!

And this the Code of my T4:

<#@ template debug="false" hostspecific="false" language="VB" #><#@ assembly name="System.Core" #><#@ import namespace="System.Linq" #><#@ import namespace="System.Xml.Linq.XDocument" #><#@ import namespace="System.Text" #><#@ import namespace="System.Collections.Generic" #><#@ output extension=".txt" #><# dim xx as new XDocument#>