Tls: first record does not look like a TLS handshake

a. System environment:

Ubuntu 20.04.1 LTS (GNU/Linux 5.4.0-45-generic x86_64)
go version go1.15.1 linux/amd64

3. The problem I’m having:

r := mux.NewRouter()
r.HandleFunc("/", HomeHandler)
...
err := certmagic.HTTPS([]string{"something.com"}, r)
...

4. Error messages and/or full log output:

TLS handshake error from <ip:port>: tls: first record does not look like a TLS handshake

This just means that the client sent a non-TLS request to a TLS server. Usually, the client is trying HTTP but the server is expecting HTTPS.

I thought maybe if there is something I could do to clear this error message, but I don’t have the client.
Thanks.