WP, Geeklog and Pages Not Found

I currently have a website which is running on Geeklog with an installation of WordPress in a subfolder. Everything seems okay, except the Page Not Found error for any page on the site now defaults to WP’s Page Not Found page.

The main site is at https://www.e-easel.org
The WP site is at https://www.e-easel.org/artifice/
If you go to a non-existant page at the main site (e.g. https://www.e-easel.org/testing404.html), you will get the WP error page.

How can I change the setup so that the error page does not default to WP’s?

Thanks!