I had quite similar problem with DropDownList . I was trying to get the selected item by the user like this :
MsgBox(ComboBox4.SelectedItem()) and always throw an exception but when I changed the dropping style to ropDown it works . at the end I used this :
MsgBox(ComboBox4.Text) along with DropDownList property and worked fine . I dunno if this would give you any clue or not !