From the Core Rules:
Save of ‘-’
Some models have a Save of ‘-’. A Save of ‘-’ means that you must roll a 7 or more. In most cases this will be impossible, so no roll need be made, but sometimes modifiers will allow you to modify a save roll so that a result of 7 or higher is possible, in which case you can attempt to make the save roll.
Of course that may change with the imminent release of 3rd edition.