The reason for the error is the passing of arguments by value to FPScript function formulas. So the argument sig soesn’t hold the object MySignal, it just gets its value contained in. This will change in FlexPro 10.
If you want to access the object in FlexPro 9, you have to pass the name and access the properties via the indirection operator. E.g. $”MySignal”$.FullName will give you the full path.