Cannot implicitly convert type to System.Guid

Kode 

        string pest = Membership.GetUser().ProviderUserKey.ToString();

        DataClasses2DataContext db = new DataClasses2DataContext();

        var newFarm = new TechFarm()
        {
            FarmName = TextBoxFarmName.Text,
            FarmAddress = TextBoxFarmAddress.Text,
            FarmPostalCode = TextBoxPostalCode.Text,
            FarmCity = TextBoxCity.Text,
            FarmCountry = System.Int32.Parse(ListBoxCountries.SelectedItem.Value),
            UserId = pest,
        };
        db.TechFarms.InsertOnSubmit(newFarm);
        db.SubmitChanges();