Warning: filesize() [function.filesize]

Hello,

I have installed Follow in my root install and am running it across multiple sites. It is running fine on my test site (with only the minimal modules installed). But one of my live sites, I keep getting the following message:

Warning: filesize() [function.filesize]: stat failed for 1 in follow_css() (line 340 of /path/to/drupal-7/modules/follow/follow.module).

The line in follow.module that is referenced is:

file_transfer($destination, array('Content-Type' => 'text/css', 'Content-Length' => filesize($destination)));

My first thought was that maybe Follow wasn't compatible with one of the other modules installed on my live site, but I set up my test site to have all the same modules running as the live site, and I still have no problems with the test site.

Does anyone have any idea what could be causing the problem? I have been using Follow since Drupal 6, but I can't get it work properly on 7.

Thanks as always.

Josh