Difficult to know why we can't find signing_key_fingerprint for a PPA

[Jo-Erlend Schinstad]

I've created a PPA, but I haven't uploaded anything yet. On my development machine I tried to add the PPA using apt-apt-repository and I got the following error message:
«Error: can't find signing_key_fingerprint at https://launchpad.net/api/1.0/~joerlend.schinstad/+archive/testing»

I thought I'd done something wrong, so I asked in #Launchpad where it was explained to me that keys aren't generated until the first upload. That sounds reasonable, but I think it should be easier to know why add-apt-repository fails. I also think it should be explained in pages like this one: https://help.launchpad.net/Packaging/PPA/InstallingSoftware

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Graham Leggett (minfrin)]

9 years later and this bug is still unfixed when building from Bionic.

The error

Error: signing key fingerprint does not exist
Failed to add key.

might be a statement of fact, but it doesn't tell me what I must do, or whether my system is broken or not, or what action I must take.