Having trouble enabling TLSv1.3 on httpd(8)

According to my understanding of the manpages (specifically
httpd.conf(5) and tls_config_set_protocols(3)), setting up TLSv1.3
should be just as easy as adding:

tls {
protocols "TLS_PROTOCOL_TLSv1_3"
}

to the appropriate server in /etc/httpd.conf . But when I do this,
httpd(8) doesn't come up. Clearly I'm not setting the right variable,
but what is that variable to set?

- Parker