Question : Combo Box  List index

I have a subform query  that uses 3 combo boxes on that form as filters. I need to have a button that will clear the selections in the  3 combo boxes so that the query  will not be filtered and the user can "start over". I tried " combobox1.value =""  " but that doesn't work. I need to set the combox.listindex to -1 to clear it however it won't let me put " combobox1.listindex=-1 " under click event for the button. Is there a fuction  listindex that will set the combo box to -1 i.e no selection?
Thanks

Answer : Combo Box  List index

combobox1.value = Null
(for all three)