Selection not persisting

[C]

I have a RadComboBox that gets its data set in the FrameWorkInitialize method like this:

 if (!IsPostBack)
            {
                string firstFilterName = adaptationFilters[0].FilterCategoryName;
                var firstFilterGroup = adaptationFilters[0].FilterCategoryChoices.Select(afc => new ComboBoxItem(firstFilterName, afc)).ToList();
                firstFilterGroup.Insert(0, new ComboBoxItem(firstFilterName, firstFilterName));
                _filterDropDown1.DataSource = firstFilterGroup;
                _filterDropDown1.DataBind();
                _filterDropDown1.SelectedIndex = 0;
}

(where ComboBoxItem is an internal class that has an object, value and a string, text)

Everything looks good until I pick a value. Well, the SelectedIndexChanged event also fires like it should, but after the postback, item 0 is selected again. This happen both with ViewStateMode enabled and without.

What am I doing wrong?