how do I delete "reachable at" email address on Apple ID?

On my Apple ID settings, there is an email address is the "reachable at" field that I want to delete but there is no option do to so. This is not the primary email for the account, but the secondary one I filled in when the account was set up years ago. Now that email address is no longer valid, so I want to delete it. I am able to ADD an another email address, but I cannot delete the one I no longer want to use. Any help appreciated. thx.