Error 1015 when trying to restore iPhone 3G stuck in recovery mode

I have an old iPhone 3G stuck in recovery mode, which I am trying to restore iOS on. But when I restore through iTunes, at the end of the restore Error 1015 shows. Furthermore, the iPhone is stuck in recovery mode, which I am unable to exit from every time I turn on the iPhone. Any answer would be greatly appreciated, as I want to get the iPhone 3G back into working condition.