how to avoid first item selection by default in wpf Listbox?

I want to avoid first item selection that happens by default in Listbox when application loads. I tried with

SelectedIndex= -1

but it did not work. I do not want any item to be selected when ListBox loads until user selects any item from Listbox. I have IsSynchronization set to true but making it false also do not solve my problem. Though i have to keep IsSychronization set to true always. But thats ok. i did google but could not find any relevant answer.

Can anyone tell me how to do this ?