GridViewComboBoxColumn AutoCompleteMode

I'm creating a combobox column as below

            Dim col As GridViewComboBoxColumn = DirectCast(grdMenuItems.Columns("item_id"), GridViewComboBoxColumn)
            col.DataSource = proc.FillDataTable()
            col.DisplayMember = "display_member"
            col.ValueMember = "value_member"

However when I try either of these

            col.AutoCompleteMode = AutoCompleteMode.SuggestAppend
            col.DropDownStyle = Telerik.WinControls.RadDropDownStyle.DropDownList
I get this 

Object reference not set to an instance of an object. Any ideas?

Later

Art