Setting ModelState values in custom model binder

I am using custom model binder in ASP.NET MVC 2 that looks like this:

    public override object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext)
    {
        if (controllerContext == null)
        {
            throw new ArgumentNullException("controllerContext");
        }
        if (bindingContext == null)
        {
            throw new ArgumentNullException("bindingContext");
        }

        BaseContentObject obj = (BaseContentObject)base.BindModel(controllerContext, bindingContext);
        if(string.IsNullOrWhiteSpace(obj.Slug))
        {
            // creating new object
            obj.Created = obj.Modified = DateTime.Now;
            obj.ModifiedBy = obj.CreatedBy = controllerContext.HttpContext.User.Identity.Name;
            // slug is not provided thru UI, derivate it from Title; property setter removes chars that are not allowed
            obj.Slug = obj.Title;
            ModelStateDictionary modelStateDictionary = bindingContext.ModelState;
            modelStateDictionary.SetModelValue("Slug", new ValueProviderResult(obj.Slug, obj.Slug, null));
...

When I get back from this binder into controller action, my business object that is provided as a parameter to the action is correctly altered (the lines obj.Created = .... work).

However, the ModelState is not updated. I know this because I have Required on my business object's Slug property and although I altered ModelStateDictionary in my custom model binder, providing a Slug to it (as you can see above), the ModelState.IsValid is still false.

If I put ModelState["Slug"] in my Watch window in Debug session, it says it has Errors (1), so apparently it is empty and as such fails.

How can I correctly alter the ModelState inside the custom model binder code?