I don't want to successfully reach your bullshit ad host. I don't want to get successfully served an ad instead of timing out. I just want to fail.
Any other behavior is wrong.
I assume you ask this honestly.
The problem isn't something like "Oh, hey, well, there's some empty space so let's setup a lemonade stand until someone buys up the property." The address is supposed to be valid, or fail fast. It is of much greater utility to everyone (except the ad farmers) to fail fast.