FormsAuthentication.Encrypt with xml userData returns null

I am trying to put xml into the user Data of a forms auth ticket. However when I go to Encrypt it the return is null.
  FormsAuthenticationTicket authTicket = new FormsAuthenticationTicket( 1, userName, DateTime.Now, DateTime.Now.AddMinutes(60), true, "<?xml version=\"1.0\" encoding=\"utf-16\"?><User xmlns:xsi=\"http://www.w3.org/2001/XMLSchema-instance\" xmlns:xsd=\"http://www.w3.org/2001/XMLSchema\" Id=\"D15B3591-98D1-41C5-AC6E-1AF7FDECC667\" UserName=\"testuser1\"> <BusinessUnit> <Roles RoleId=\"76E69AE7-B793-47B2-8016-06D1F727DA54\" RoleName=\"Company Administrator\"> <ActionLevel Name=\"Company\"><SiteArea><AreaActions Id=\"7540C995-E9E6-44D2 ...