Avatars plugin throwing errors in log, but no problems for users

My error log is reporting a lot of this:
PHP Notice: Undefined index: HTTP_IF_MODIFIED_SINCE in /var/www/vhosts/blogs.pnwlocalnews.com/httpdocs/wp-content/avatar.php on line 142

... but users have reported no problems with the avatar plugin. My users are mostly set to the "author" role. Avatars, both author and blog, are showing up on the site just fine.

I'm running Avatars 3.4.0 on MU 2.8.4a.

Any help would be appreciated. Thank you.