Trouble Getting Comments RSS to Work

Using WP 1.5 (not sure what version), My rss2 feed works fine, but I can’t get my comments RSS feed to work. When I put it in the w3c validator, I get the following message:

This feed does not validate.

* Server returned HTTP Error 404: Not Found
(https://validator.w3.org/feed/check.cgi?url=http%3A%2F%2Fsongoose.com%2Fwp-commentsrss2.php)

Can anyone help me here?

Thanks