"Empty argument. Try again" while trying to add a new key

[baobaab]

On a fresh dokku setup (Ubuntu 18.04, dokku 0.19.12), I was not able to add either a shared key or an app key, I always get the message Empty argument. Try again.

Here is the test I did with "github.com" :

root@u18-dokku:~# dokku apps:create test
-----> Creating test... done
root@u18-dokku:~# dokku hostkeys:shared:add "$(ssh-keyscan -t rsa github.com 2>/dev/null)"
Empty argument. Try again.
root@u18-dokku:~# dokku hostkeys:app:add test "$(ssh-keyscan -t rsa github.com 2>/dev/null)"
No app specific keys folder available. creating..
No app specific keys file available. creating..
Empty argument. Try again.