getting UUID "000000" instead of real UUID string

Getting the uuid this way:

let identifier = UIDevice.current.identifierForVendor?.uuidString ?? ""

some devices return "000000" instead the real uuid string. I need that string to identify the app instance installed, but this way I'm getting some device having the same "000000" string.

Am I doing something wrong in the way I'm getting the uuid?
getting UUID "000000" instead of real UUID string
 
 
Q