Clear Selections of a Combobox
I faced the same problem and possibly we have the same scenario.
I understand that you don’t want to run clear method and make the current listed items vanish, but just make the combo having no value.
In case just before the attempt of making combo value null or blank string ("") or ListIndex=-1 you had repopulated the combo with new set of items this might happen. Before repopulating the combo with new items, you should call the clear method and then repopulate the combo with the new items and after that set the value to null or blank string ("") value.