SSL doesn't activate in Mongrel

I have an app I’ve been working on for a while. I just tried to
activate SSL for it. I followed the instructions found at
http://wiki.rubyonrails.com/rails/pages/HowToEnableSSL. Everything
seemed to install/configure OK, no errors. But I now find that nothing
is listening on port 443. “Connection refused”

So what might be the problem?