Server UI user (server-control-username) hardcoded to "kopia"?

I am starting kopia server via systemd with the following ExecStart-Line:

/usr/bin/kopia server start --address 0.0.0.0:5000 --tls-cert-file /etc/kopia/kopia.cert --tls-key-file /etc/kopia/kopia.key --config-file /etc/kopia/repository.config --server-control-username kopia --server-control-password somepassword

Specifiying any other user than “kopia” does not work. Why?