How to pass the value of the @Html.DropDownList to the controller?

I have a modal register that I get 2 values, I use an AJAX to save the information, however the value of @Html.DropDownList does not go to my Controller, the strange is the value of the Description arrives in the controller.

<div class="modal-body">
            <div class="form-horizontal">
                @Html.ValidationSummary(true)

                <div class="form-group">
                    @Html.Label("Descrição", new { @class = "control-label col-md-3" })
                    <div class="col-md-9">
                        @Html.TextBoxFor(model => model.Descricao, new { @class = "form-control" })
                        @Html.ValidationMessageFor(model => model.Descricao)
                    </div>
                </div>
                <div class="form-group">
                    @Html.Label("Tipo de Cálculo", new { @class = "control-label col-md-3" })
                    <div class="col-md-9">
                        @Html.DropDownList("TipoCalculo")
                    </div><br />
                </div>

            </div>
        </div>

<script>
$(document).ready(function () {       
    //$("#AjaxPost").click(function () {
    //alert($('#TipoCalculo :selected').val())

    $("#AjaxPost").click(function () {
        var dataObject = {
            Descricao: $("#Descricao").val(),
            TipoCalculo: $('#TipoCalculo :selected').text(),
        };

        $.ajax({
            url: "@Url.Action("SalvarMilestone", "Dashboard")",
            type: "POST", 
            data: dataObject,
            dataType: "json"
        });
    });
});