there was an error in generating xml document

Hello Mate,

I am calling a web method from my code behind and i got this error
there was an error in generating XML document .and the web method is not getting called.

i am using Asp.net2.0 Ajax extentions and my code is

in mark up:

<ajax:UpdatePanel ID="Updatepanel_Add_New_Client1" runat="server" UpdateMode ="conditional" >
<ContentTemplate >
<table id="table1" width ="800px" >
<tr>
<td style="width: 200px; height: 26px;" >
<asp:Label ID="lblFName" runat="server" Text="First Name"></asp:Label></td>
<td style="width: 200px; height: 26px;" >
<asp:TextBox ID="txtFname" runat="server" Width=150px></asp:TextBox></td>
<td ...