CGI with nginx getting 502 bad gateway error

Question

I want to use my RPi to source some data wirelessly. I'm working from Derek Molloy's Exploring Raspberry Pi. Step 1: get nginx working. I wrote my own index.html file and can get to it over my local LAN. So nginx works. Now I want to add cgi capability so, eventually, I can write a C or C++ program to control a device and return data.

So I followed the instructions on P. 490-491 of Molloy's book, adding fcgiwrap and adding the necessary include line in Nginx default. I typed in the simple cgi script with nano. ./test.cgi gives the expected result. So when I query:

192.168.1.135/cgi-bin/test.cgi

the response is: 502 bad gateway

This has to be a brain-dead simple problem, but I can't find anyone with exactly this issue. 502 bad gateway with Apache? Sure. But what 3 characters or directory settings or other simple stuff do I need to check? Thanks.

Nginx config file (with most commented-out lines removed):

server {
    listen 80 default_server;
    include /etc/nginx/fcgiwrap.conf;
    listen [::]:80 default_server;

    # root ~/data/www;
    root /var/www/html;

    # Add index.php to the list if you are using PHP
    index index.html; 

    server_name _;
}