Trouble w/ Certbot & Let's Encrypt on Nginx/Ubuntu14.04

I’m following these instructions to install Certbot and Let’s Encrypt on my droplet running Nginx and Ubuntu 14.04, but am hitting a snag.

When I get to the alpha plugin portion: certbot --nginx I get the following response:

    Saving debug log to /var/log/letsencrypt/letsencrypt.log
    No names were found in your configuration files. Please enter in your domain
    name(s) (comma and/or space separated)  (Enter 'c' to cancel):  

I enter my domain name, hit enter and get:

    Obtaining a new certificate
    Performing the following challenges:
    tls-sni-01 challenge for **MYDOMAINNAME**
    Cleaning up challenges
    Cannot find a VirtualHost matching domain **MYDOMAINNAME**.

I’m not sure how to get past this error. Here’s my sites-available server block:

    server {
    	listen 80 default_server;
    	listen [::]:80 default_server ipv6only=on;
    	
    	root /usr/share/nginx/html;
    	index index.html index.htm;
    
    	# Make site accessible from http://localhost/
    	server_name localhost;
    
    	location ~ /.well-known {
    		allow all;
    		root /usr/share/nginx/html;
    	}
    
    	location / {
    		
    		# First attempt to serve request as file, then
    		# as directory, then fall back to displaying a 404.
    		try_files $uri $uri/ =404;
    		# Uncomment to enable naxsi on this location
    		# include /etc/nginx/naxsi.rules
    	}
    }

Any ideas where I’m going wrong?