I think there isn't an element with index -1, all index for select are >0
there is an element in Combobox which has empty text right?
if you debug this which position does this element have in comboBox.Items?
This is the index you should choose.
Or if the items has Automation Id's use the longer way with
ListItem item = comboBox.Items.Where(p=>p.AutomationElement.Current.AutomationID == "correct id").First();