Clean method of providing blank first value in Silverlight Combobox?

I've got a combobox in silverlight that is bound to a list of objects in XAML coming from RIA servcies. Is there a开发者_JS百科 clean method to add a blank value for the first item so that users have to click the dropdown in order to select something? Right now, it just displays the first item, even if the selected index is set to -1.