Error sending JSON object to asp.net WebMethod, using jQuery

I get an error of "Cannot convert object of type System.String' to type 'System.Collections.Generic.List'1[...+ContactHolder]'"

This is my C# code:

public class ContactHolder{
    public String Name { get; set; }
    public String Address { get; set; }
}

[WebMethod]
public static string AddContact(Object items){
    List<ContactHolder> contacts = new JavaScriptSerializer().ConvertToType<List<ContactHolder>>(items);
    return String.Format("You added {0} contacts!", contacts.ToString());
}

This is my Javascript code (note I have included jquery-1.4.1.js and json2.js in my project)

function Test() {
var arr = new Array();
var contact = new Object();
contact.Name = "Ben";
contact.Address = "Ohio";

var contact2 = new Object();
contact.Name = "Jon";
contact.Address = "Illinois";

arr[1] = contact;
arr[2] = contact2;

var DTO = JSON.stringify(arr);
PageMethods.AddContact(DTO, OnMyMethodComplete);}

function OnMyMethodComplete(result, userContext, methodName) {alert(result);}

When I pass DTO into myPageMethod it looks like this: "[{"Name":"Ben","Address":"Ohio"},{"Name":"Jon","Address":"Illinois"}]"

When it appears in my WebMethod as items it looks like this: "[{\"Name\":\"Ben\",\"Address\":\"Ohio\"},{\"Name\":\"Jon\",\"Address\":\"Illinois\"}]"

The code errors on the call to JavaScriptSerializer().ConvertToType, the message is {"Cannot convert object of type System.String' to type 'System.Collections.Generic.List'1[...+ContactHolder]'"}

What do I need to do to make this work?