Acrobat Combo Box: Blank Default

When setting up a combo box in Acrobat 6.0, I want the default item which appears before the user makes a drop-down selection to be blank. I will have a series of identical combo boxes on a form and want them lines to print blank unless the user chooses an item from the list to populate the box. Under Options in combo box properties, I cannot add a blank item. The Add button grays out. Thus, I can't get a default blank item.

Thanks for your help.