Error consuming Web Service ASP.NET

So, I'm studying Web Services in ASP.NET and I have a doubt. I have two methods: Add (that does the sum of two numbers) and GetCalculations(that displays the latest calculations, through EnabledSession property. When I open my .asmx on browser, it shows the sum and the latest calculations in a xml file, but, when I open my Web Form on browser, I can do the sum,but the calculations are not displayed.Can anyone help me, please? These are my codes:

WebForm1.aspx.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

namespace CalculatorWebApplication
{
    public partial class WebForm1 : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {

        }

        protected void btnAdd_Click(object sender, EventArgs e)
        {
            CalculatorService.CalculatorWebServiceSoap client =
                new CalculatorService.CalculatorWebServiceSoapClient();
            int result = client.Add(Convert.ToInt32(txtFirstNumber.Text),
                Convert.ToInt32(txtSecondNumber.Text));
            lblResult.Text = result.ToString();

            gvCalculations.DataSource = client.GetCalculations();
            gvCalculations.DataBind();

            gvCalculations.HeaderRow.Cells[0].Text = "Recent Calculations";
        }
    }
}

WebForm1.aspx

<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="WebForm1.aspx.cs" Inherits="CalculatorWebApplication.WebForm1" %>

<!DOCTYPE html>

<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <title></title>
</head>
<body>
    <form id="form1" runat="server">
    <table style="font-family:Arial">
        <tr>
            <td>
                <b>First Number</b>
            </td>
            <td>
                <asp:TextBox ID="txtFirstNumber" runat="server"></asp:TextBox>
            </td>
        </tr>
        <tr>
            <td>
                <b>Second Number</b>
            </td>
            <td>
                <asp:TextBox ID="txtSecondNumber" runat="server"></asp:TextBox>
            </td>
        </tr>
        <tr>
            <td>
                <b>Result</b>
            </td>
            <td>
                <asp:Label ID="lblResult" runat="server"></asp:Label>
            </td>
        </tr>
        <tr>
            <td colspan="2">
                <asp:Button ID="btnAdd" runat="server" Text="Add" OnClick="btnAdd_Click" />
            </td>
        </tr>
        <tr>
            <td colspan="2">
                <asp:GridView ID="gvCalculations" runat="server">

                </asp:GridView>
            </td>
        </tr>
    </table>
    </form>
</body>
</html>

CalculatorWebService.asmx.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Services;
using System.Web.Services.Protocols;

namespace WebServicesDemo
{
    /// <summary>
    /// Summary description for CalculatorWebService
    /// </summary>
    [WebService(Namespace = "http://pragimtech.com/webservices")]
    [WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
    [System.ComponentModel.ToolboxItem(false)]
    // To allow this Web Service to be called from script, using ASP.NET AJAX, uncomment the following line. 
    // [System.Web.Script.Services.ScriptService]
    public class CalculatorWebService : System.Web.Services.WebService
    {

        [WebMethod(EnableSession = true)]
        public int Add(int firstNumber, int secondNumber)
        {
            List<string> calculations;
            if (Session["CALCULATIONS"] == null)
            {
                calculations = new List<string>();
            }
            else
            {
                calculations = (List<string>)Session["CALCULATIONS"];
            }

            string strRecentCalculation = firstNumber.ToString() + " + " 
                + secondNumber.ToString() + " = " 
                + (firstNumber + secondNumber).ToString();
            calculations.Add(strRecentCalculation);
            Session["CALCULATIONS"] = calculations;

            retu firstNumber + secondNumber;
        }

        [WebMethod(EnableSession = true)]
        public List<string> GetCalculations()
        {
            if (Session["CALCULATIONS"] == null)
            {
                List<string> calculations = new List<string>();
                calculations.Add("You have not performed any calculations");
                retu calculations;
            }
            else
            {
                retu (List<string>)Session["CALCULATIONS"];
            }
        }
    }
}