ImageAssist looking for zen.css in subtheme

I'm getting a page not found when using ImageAssist on a site with a Zen subtheme. Version details as follows:
Drupal 6.6
Zen theme 6.x-1.0-beta3
Image 6.x-1.0-alpha3
Image Assist 6.x-1.0

Zen sub-theme is called 'pastoral' in folder sites/all/themes/pastoral. There is no file called zen.css in this folder.

I get a log entry of:
page not found 20 Nov 2008 - 3:26pm sites/all/themes/pastoral/zen.css
when I use ImageAssist.

I'm looking for assistance to understand how to correct this behaviour. Is it an issue more relating to ImageAssist than to Zen? Should I create an empty file "sites/all/themes/pastoral/zen.css" even though it's not supposed to be required for D6?

Thanks for any help.