Leaving child select list blank doesn't work

I am using an exposed filter in a view. In my vocabulary there are provinces and towns. In the filter, I want let the user to choose any of both. So, the two dropdowns have a blank option. The first time I select a province and leave the town blank, the filter works right. But if I filter by town, and later I select the blank option of towns, then the filter doesn't work. The page is refreshed and it returns to the previous status, where a town is selected and its result is shown.
Any help please?