ASP.NET MVC UpdateModel for foreign key dropdownlist

Question

Using MVC 1.0 I have the following custom form view model

public class NoteFormViewModel
{
    // Properties
    public Note Note { get; private set; }
    public SelectList PrivacyOptions { get; private set; }

    // Constructor
    public NoteFormViewModel(Note note)
    {
        SharePermissionsRepository sharePermissionsRepository = new SharePermissionsRepository();
        var sharePermissions = sharePermissionsRepository.FindAllSharePermissions();
        Note = note;
        PrivacyOptions = new SelectList(sharePermissions, "id", "share_permission", note.share_permission_id);
    }
}

and two DB tables. Notes and SharedPermissions. Notes.shared_permission_id (integer) is a foreign key to SharedPermissions.id (integer, primary key, identity)

The edit view is bound to NoteFormViewModel and the dropdown list is bound as follows.

<%= Html.DropDownList("share_permission_id", Model.PrivacyOptions)%>

Then on submit of the form i do this.

[AcceptVerbs(HttpVerbs.Post)]
[ValidateInput(false)]
public ActionResult Edit(int id, FormCollection formValues)
{
    Note note = notesRepository.GetNote(id);

    try
    {
        UpdateModel(note);
        notesRepository.Save();
        return RedirectToAction("Details", new { id = note.id });
    }
    catch
    {
        ModelState.AddModelErrors(note.GetRuleViolations());

        return View(new NoteFormViewModel(note));
    }
}

But the note.shared_permission_id is not updated, even though the forms collection contains a key of "shared_permission_id" and the correct updated value. I do not get any exceptions.

If however i explicity set the note.share_permission_id like this.

UpdateModel(note);
note.share_permission_id = System.Convert.ToInt32(Request.Form["share_permission_id"]);
notesRepository.Save();
return RedirectToAction("Details", new { id = note.id });

Then the share_permission_id is updated correctly. I've trawled some of the other posts on here and i can see how they are related to this but, not quite close enough for me to understand exactly what is going on. What's going on? Is this a bug in the UpdateModel method or is it something else completely?