Selected Dropdownlist value not being passed to MVC model using Ajax.Beginform

Question

I've used code very similar to what follows elsewhere in my program. It works there. Why, when I click submit, does my selected item in the dropdownlistfor NOT bind to the model?

public class APIDataViewModel
    { 
        public ChangeBuildState ChangeBuildStateValue { get; set; }
        public int SelectedIntValue { get; set; }
    }

 public class ChangeBuildState
    {
        public Dictionary<int, string> BuildState { get; set; }

        public ChangeBuildState()
        {
            BuildState = new Dictionary<int, string>()
            {
                { -1, "Halt"},
                { 4, "Ready"}
            };

        }
    }

@using (Ajax.BeginForm(
                            "ChangeBuildStateRun",
                            "Stores",
                            new
                            {
                                Area = "",
                                locationNumber = Model.locationNumber,
                                macAddress = Model.mac,
                                buildState = Model.SelectedIntValue
                            },
                            new AjaxOptions
                            {
                                UpdateTargetId = "APIModal",
                                InsertionMode = InsertionMode.Replace,
                                HttpMethod = "POST",
                                OnSuccess = "CloseAddWkstnModals",
                                OnFailure = "APIFailed"
                            },
                            new
                            {
                                id = "ChangeBuildStateForm"
                            }))

                {
                    <div class="row Padding_Std center-block">
                        <div class="col-sm-7 col-sm-offset-4 col-md-7 col-md-offset-2 Padding_Std">
                            <label class="ModalInputLblStyle">IP Address: </label>
                            @Html.EditorFor(m => m.ipaddress,
                             new
                             {
                                 htmlAttributes = new
                                 {
                                     @class = "input-sm AddEditWorkstationTBoxStyle",
                                     @readonly = "readonly",
                                     @disabled = "disabled"
                                 }
                             })
                        </div>
                    </div>
                    <div class="row Padding_Std center-block">
                        <div class="col-sm-7 col-sm-offset-4 col-md-7 col-md-offset-2 Padding_Std">
                            <label class="ModalInputLblStyle">MAC Address: </label>
                                @Html.EditorFor(m => m.mac,
                                new
                                {
                                    htmlAttributes = new
                                    {
                                        @class = "input-sm AddEditWorkstationTBoxStyle",
                                        @readonly = "readonly",
                                        @disabled = "disabled"
                                    }
                                })
                        </div>
                    </div>
                    <div class="row Padding_Std center-block">
                        <div class="col-sm-7 col-sm-offset-4 col-md-7 col-md-offset-2 Padding_Std">
                            <label class="storeSearchInputLblStyle">Select Build State: </label>
                                @Html.DropDownListFor(m => m.SelectedIntValue,
                                    new SelectList(
                                    Model.ChangeBuildStateValue.BuildState,
                                    "Key",
                                    "Value"),
                                    "--- Select ---",
                                    new { @class = "DDL_LogLevelStyle" })
                        </div>
                    </div>
                    <div class="row">
                        <div class="col-sm-9 col-sm-offset-4">
                            <div class="col-md-4 col-sm-3 center-block" id="BtnChangeBuildState">
                                <input type="submit" value="Change Build State" class="btn btn-sm button-alt-1 button-v center-block" />
                            </div>
 ...
                    </div>
                }

public ActionResult ChangeBuildStateRun(int locationNumber = 0, string macAddress = "", int buildState = 999)
        {
            APIDataViewModel model = new APIDataViewModel();
            model.locationNumber = locationNumber.ToString();
            model.mac = macAddress;
            try
            {
                bool apiCallResult = _epmService.UpdateStoreWorkstationBuildState(locationNumber, macAddress, buildState);
                model.webapi_name = "Change Build State";
                model.response_type = "ChangeBuild";
                model.success = apiCallResult;
                return PartialView("_APIResults", model);
            }
            catch
            {
                model.webapi_name = "Change Build State";
                model.response_type = "ChangeBuild";
                model.success = false;
                return PartialView("_APIResults", model);
            }
        }

I haven't included the _APIResults partial view, as buildstate always comes in as 0, when it appears it should come in as -1 or 4 (assuming you change the dropdown to Halt or Ready, which is what I did.