Avatar Plugin - PHP error in avatar.php

Hi guys,

I followed all the installation instructions in install.txt for the Avatars plugin and double checked it all. I can upload avatars to my main blog using Settings-->Blog Avatar and it displays ok in that page. However in my Apache logs I'm getting.

PHP Notice: Undefined index: HTTP_IF_MODIFIED_SINCE in /var/www/schblogs/wp-content/avatar.php on line 142,

And avatars aren't showing up on the blogs, its still generating <img> tags pointing to default-avatar-96.png etc.

Avatars version is 3.4
WPMU version is 2.8.6

let me know of any other info thats needed.