nginx – host not found in “$server_listen_port” of the “listen” directive

With the following nginx config excerpt:

map $server_name $server_listen_port {
    default 443;
    localhost. 80;
    localhost  80;
}

server {
    server_name "${ENVIRONMENT_SERVER_NAME}.";
    listen $server_listen_port http2 reuseport;
    # ...
}

I get the following error when validating the configuration with ENVIRONMENT_SERVER_NAME set in the environment to “localhost”:

nginx: [emerg] host not found in "$server_listen_port" of the "listen" directive in /etc/nginx/conf.d/99_main.conf:38
nginx: configuration file /etc/nginx/nginx.conf test failed

I am trying to decipher what this error message means exactly. Does it mean that the listen statement is invalid because it is missing a hostname?

Or is it perhaps trying to tell me there is no value in the map for the host? I would expect the “default” to be invoked in that case.

I have tried all manner of variations without really getting anywhere.

What is causing this error, and how can I fix it?