How do I get a proper string from JSON response in AJAX function in asp.net?

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ASP.NET

Ajax

Following is my ajax function:

$("[id*=DropDownListSubBrand]").change(function () {
                var subBrandval = $('[id*=DropDownListSubBrand]').val();
                alert(subBrandval);
                //create the ajax request
                $.ajax({
                    type: "POST", //HTTP method
                    url: "PCAdd.aspx/OnSubBrandChange", //page/method name
                    data: "{'Brand_Id':'" + $('[id*=DropDownListSubBrand]').val() + "'}", //json to represent argument
                    contentType: "application/json; charset=utf-8",
                    dataType: "json",
                    success: function (msg) { //handle the callback to handle response                
                        //request was successful. so Retrieve the values in the response.                        
                        alert(msg.valueOf());

                    }                 

                });
            });

Following is the function that is being called from inside the above function:

[System.Web.Services.WebMethod]
        public static string OnSubBrandChange(int Brand_Id)
        {
            Brand objBrand = new Brand();
            string strBrand=objBrand.GetBrand(Brand_Id);

            return strBrand;
        }

The response that I receving from the "OnSubBrandChange" is correct when I am seeing it with a breakpoint on the "return" statement in the above function but the "alert" startement inside "success(msg)" is returning [object Object]. How would i get a proper string from the ajax method?