$ heroku domains
=== my-app Custom Domains
Domain Name                                       DNS Target
────────────────────────────────────────────────  ──────────────────────────────────────────────────────────────
*.my-app.co.uk                            wildcard.my-app.co.uk.herokudns.com
www.my-app.co.uk                          www.my-app.co.uk.herokudns.com

$ heroku run rake letsencrypt:renew                                                                                                                                                                                          Running rake letsencrypt:renew on ⬢ my-app... up, run.5599 (Hobby)
Creating account key...Done!
Registering with LetsEncrypt...Done!
Using 5 configured Heroku domain(s) for this app...
Performing verification for *.my-app.co.uk:
rake aborted!
Acme::Client::Error::Malformed: Invalid character in DNS name