Custom Model Binder For DropDownList Not Selecting Correct Value?

i've created my own custom model binder to handle a Section DropDownList defined in my view as:

Html.DropDownListFor(m => m.Category.Section, new SelectList(Model.Sections, "SectionID", "SectionName"), "-- Please Select --")

And here is my model binder:

public class SectionModelBinder : DefaultModelBinder
{
public override object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext)
{
var value = bindingContext.ValueProvider.GetValue(bindingContext.ModelName);
if (bindingContext.ModelType.IsAssignableFrom(typeof(Section)) && value != null)
{
if (Utilities.IsInteger(value.AttemptedValue))
return Section.GetById(Convert.ToInt32(value.AttemptedValue));
else if (value.AttemptedValue == "")
return null;
}
return base.BindModel(controllerContext, bindingContext);
}
}

Now within my controller i can say:

[HttpPost]
public ActionResult Create(FormCollection collection)
{
var category = new Category();
if (!TryUpdateModel(category, "Category")
return View(new CategoryForm(category, _sectionRepository().GetAll()));
}

This validates nicely and the correct value for the section is assigned when the model is updated, however it does not select the correct value if another property doesn't validate.