I have this error : "**Invalid web service call, missing value for parameter** " when I call a web service method with a parameter.

I'm testing with a web service method without parameter that returns the same type of object and it works well.

Here is my web service method : 

```
 [WebMethod]
    [ScriptMethod(UseHttpGet = true)]
    public ResponseStatistic_3 Statistic_3(string klant)
    {
            Statistic_3[] items = Helper.Helper_Statistic_3(klant).ToArray();
            ResponseStatistic_3 response = new ResponseStatistic_3(items);

            return response;
    }

```

Here is my javascript code, I retrieve the good value in kla variable :

```
 function getStatistic3() {

var response;
var allstat3 = [];
var kla = $('#Select1').val();
var dataJSon = { klant: kla }

if (kla) {
    $.ajax({
        type: 'GET',
        url: 'http://localhost:52251/Service1.asmx/Statistic_3',
        data: dataJSon,
        contentType: 'application/json; charset=utf-8',
        dataType: 'json',
        processData: false,
        success: function (msg) {
            response = msg.d;
            for (var i = 0; i < response.Items.length; i++) {
                var j = 0;
                allstat3[i] = [response.Items[i].Interventie, response.Items[i].Sum[j], response.Items[i].Sum[++j], response.Items[i].Sum[++j], response.Items[i].Sum[++j], response.Items[i].Sum[++j]];
            }
            fillDataTable(allstat3);
        },
        error: function (e) {
            alert("error loading statistic 3");
        }
    });
} else {
    alert("statistic 3 null");
}
}

```

I'm testing too with JSON.stringify({ klant: kla }) and I have the same error.

I looked at several forums but in vain.
What's wrong?