Web Forms :: Insert TextBox And GridView Value Into DB - Error Occurred While Executing Command Definition

I have develop a function in which insert textbox value and grid view value into database.

Problem is that my web method call but it will add some time value into database and give me the following error. Its work once time other can’t work.

An error occurred while executing the command definition. See the inner exception for details.

function addData() {
debugger;
var Customers = new Array();
var desigantion = $("#TextBox4").val();
$('[id*=GridView1]').find('tr:has(td)').each(function () {
var Customer = {};

[Code] ....

My web method

public class Customerdata {
public string code { get; set; }
public string Product { get; set; }
public string Rate { get; set; }

[Code] ....

After once time run then it give me an error.

An error occurred while executing the command definition. See the inner exception for details.