The javascript code does not send to the controller

I am doing a web application in C # ASP.NET MVC5, I need in the view to validate a dropdownlist that is not empty, and when it is full send the data to the controller.

JavaScript

function Validar() {
    var pais_id = document.getElementById("id_pais ").selectedIndex;
    if (pais_id == 0) {
        document.getElementById("id_pais ").focus();
        alert('[ERROR] El campo Test es requerido.');
        return false;
    }
    else
    {
       document.registros.action = @Url.Action("Index", "Registros");
      document.registros.submit();  
    }
}

the view

@using (Html.BeginForm(null, null, FormMethod.Post, new { name = "registros", id = "registros", enctype = "multipart/form-data" }))
{
 @Html.AntiForgeryToken()

<div class="form-horizontal">
    @Html.ValidationSummary(true, "", new { @class = "text-danger" })
    <div class="form-group">
        @Html.LabelFor(model => model.pais)
        <div class="col-md-10">
            @Html.DropDownList("id_stb", null, "Seleccione un Pais")

        </div>
        @if (ViewBag.Message != null)
        {
        @ViewBag.Message
        }
    </div>

When I try only the alert if it works, but when I add the part of the submit , the method does not work anymore, not even the alert . I guess I have the URL wrong to the controller. I'm new to this from C # and asp.net, thanks in advance.