ASP.NET MVC Html.BeginForm Problem

I have a partial view:

<%@ Control Language="C#" Inherits="System.Web.Mvc.ViewUserControl<DomainModel.Entities.Product>" %>

<div class="item">
    <h3><%= Model.Name %></h3>
    <%= Model.Description %>

    <% using (Html.BeginForm("AddToCart", "Cart")) { %>
        <%= Html.Hidden("ProductID") %>
        <%= Html.Hidden("returnUrl", ViewContext.HttpContext.Request.Url.PathAndQuery) %>
        <input type="submit" value="+ Add to cart" />
    <% } %>

    <h4><%= Model.Price.ToString("c")%></h4>
</div>

And here is the html that gets rendered:

<div class="item"> 
    <h3>Kayak</h3> 
    A boat for one person
    <form action="" method="post">
        <input id="ProductID" name="ProductID" type="hidden" value="1" /> 
        <input id="returnUrl" name="returnUrl" type="hidden" value="/" /> 
        <input type="submit" value="+ Add to cart" /> 
    </form> 
    <h4>$275.00</h4> 
</div> 

Nothing happens when the submit button is clicked and I am pretty sure it's because the form action attribute has no value. Shouldn't BeginForm(action, controller) take care of rendering out the form action? What am I doing wrong?

EDIT

Code from CartController AddToCart action:

    public RedirectToRouteResult AddToCart(Cart cart, int productID, string returnUrl)
    {
        Product product = productsRepository.Products.FirstOrDefault(p => p.ProductID == productID);

        cart.AddItem(product, 1);
        return RedirectToAction("Index", new { returnUrl });
    }

EDIT 2

The view that renders the partial:

<asp:Content ID="Content2" ContentPlaceHolderID="MainContent" runat="server">
    <% foreach (var product in Model) { %>
        <% Html.RenderPartial("Pr