ComboBox help

I have 3 check boxes and a combobox. By default, if someone chooses an item from the combobox without one of the 3 checkboxes being selected, it throws an error and tells them to first pick a checkbox, then to repick from the combo box. That works fine as long as they don't pick the same item from the combobox. What I would like is a way to reset the combobox to the default value if I throw the error (that way if the user immediately goes back and picks the same option under the combo box (without having to switch to another item first) then it will work).

Thanks in advance.