MVC中的Ajax错误

Question

My ajax is

<script>
    jQuery(function ($) {                               
        var size = $('#ddlsize').val();
        var color = $('#ddlcolor').val();    
        $('#addtocart').click(function () {
            $.ajax({
                url: '@Url.Action("AddTocart", "ShoppingCart")',
                data: {                                             
                    id:  '@Model.ProductId',
                    size: size,
                    color: color,                                           
                },
                dataType: "html",
                type: 'POST',
                success: function (data) {
                    alert("Da them vao gio hang");
                },
                error: function () {
                    alert("Co loi xay ra vui long thu lai");
                }
            });
        });
    });
</script>

And in Controller

public ActionResult AddTocart(int id, string size, string color)
{
    Product productitem = dbcon.Products.Where(p => p.ProductId == id).SingleOrDefault();
    var cart = ShoppingCart.Getcart(this.HttpContext);
    cart.AddtoCart(productitem, size, color);
    return RedirectToAction("Index");
}

Some time when i click in button add to cart,i see the error but it has been do an action and add to the cart.i didn't how it add product to cart when error