Are you unable to show a Duke ID because you lost it or you have no association with Duke whatsoever?

Be very careful about using any codes that you aren't eligible for. There was a thread here from another renter who used a code he wasn't eligible for and he was threatened by corporate security to return the car immediately and was banned from National for life.

Also if you get into an accident and if they find out you provided any misleading information it could void any insurance or coverage that National or your credit card company provides.