error_page and named locations

[cynix]

I have the following config on 0.7.26:

server {
 ...

 location / {
  error_page 404 = @cms;
 }

 location @cms {
  fastcgi_pass 127.0.0.1:1234;
  include fastcgi_params;
  fastcgi_param SCRIPT_FILENAME /www_root/index.php;
  fastcgi_param SCRIPT_NAME /index.php;
 }

 location ~ \.php$ {
  fastcgi_pass 127.0.0.1:1234;
  include fastcgi_params;
  fastcgi_param SCRIPT_FILENAME /www_root$fastcgi_script_name;
  fastcgi_param SCRIPT_NAME $fastcgi_script_name;
  fastcgi_intercept_errors on;
  error_page 404 = @cms;
 }

 ...
}

This should pass all requests to non-existent paths to /index.php so "clean
URLs" will work and the CMS will display the corresponding pages. Indeed it
works if I try to access a URI that is first handled by the "location /" block,
such as http://my.com/2008/12/my-blog-post.

However, when I access a URI that is first handled by the "location ~ \.php$"
block, such as http://my.com/non-existent.php, theoretically nginx will first
pass this request to FastCGI, then PHP sends back 404 because it can't find the
non-existent.php file, then nginx should honor the error_page directive and send
the request to @cms. In reality it results in a 502 Bad Gateway. The error log
says "upstream sent invalid header while reading response header from upstream".
But if I used the following:

 location ~ \.php$ {
  
  error_page 404 = /index.php;
 }

Then it will work as intended.

Why can't I use a named location for error_page if the location block contains a
fastcgi_pass directive? (I assume this is the case for proxy_pass too, but I
didn't test that one.)