Label control value disappeared

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ASP.NET

Javascript

Hi,

I have 2 textboxes ,one label control and one button on aspx page. I want to display result of two textboxes on the label control on buttons onclientclick event.

I have one web method and I am calling this method from javascript function.

Problem is that when I assign result(ie sum) in the label control then it visible only for 1 second even less than that and disappears. Even no server event is being fired after that.
can you please help.
My code is given below:

  <script type="text/javascript">
        function CallSum() {
            var Num1 = $get('TextBox1');
            var Num2 = $get('TextBox2');
            var lblControl = $get('lblResult');

            //call server side function
            PageMethods.Sum(Num1.value, Num2.value, CallSuccess, CallFailed, lblControl);
        }
          function CallSuccess(result, lblControl) {
            //Show the result in lblresult
                lblControl.innerHTML = result;
           }

        function CallFailed(result, lblControl) {
            if (result !== null) {
                alert(result.get_message());
            }
        }
    </script>
First Number<asp:TextBox ID="TextBox1" runat="server"></asp:TextBox><br />
   Second Number<asp:TextBox ID="TextBox2" runat="server"></asp:TextBox><br />

   Result = <asp:Label ID="lblResult" ClientIDMode="Static" runat="server"></asp:Label><br />
   <asp:Button ID="button1" runat="server" Text="Get SUM" OnClientClick="return CallSum();" />

and server method is:

[System.Web.Services.WebMethod]
        public static int Sum(int n1, int n2)
        {
            try { return n1 + n2; }

            catch (Exception ex) { throw ex; }
        }

Thanks in advance.
Vivek.