WP Ignores 404.php template

I have a fresh install of WP 2.1 running on an Apache webserver at pair networks. I am not using mod_rewrite or custom URLs, just the default:
https://www.example.com/?p=226

I am using a custom theme that includes a 404.php file with my custom error page. If I manipulate the URL with a non-existant post, e.g.

https://www.example.com/?p=22020

WordPress uses the error section that is embedded in the single.php template, e.g.


<?php else : ?>
<h2 class="center">Not Found</h2>
<p class="center">Sorry, but you are looking for something that isn't here.</p>
<?php include (TEMPLATEPATH . "/searchform.php"); ?>


and it appears to be ignoring the 404.php template. Is there a way to get it to use the 404.php template for all errors?