Why does curl -k -I https://host.example.com not return response headers if certificate is not valid?

Does anyone know why curl with -k (--insecure) option and -I for show headers still shows the html response and not the headers as expected? Working as expected: $ curl -I https://validsslcert.example.com HTTP/1.1 302 Moved Temporarily Server: Apache-Coyote/1.1 ... $ curl -k -I https://validsslcert.example.com HTTP/1.1 302 Moved Temporarily Server: Apache-Coyote/1.1 ... $ curl -k https://invalidcert.example.com
... NOT working as expected: $ curl -k -I https://invalidcert.example.com

Maintenance

It doesn't really matter here what I'm doing, but I'm testing what headers get set to identify different backend acl logic on haproxy. I would expect curl to allow me to make an insecure connection (invalid certificate) and still return the headers?