[Plugin: AdRotate] The plugin generated 55 characters of unexpected output during activation

I got this message at the top of the admin screen when activating the plugin in WP 3.3 RC2

The plugin generated 55 characters of unexpected output during activation. If you notice “headers already sent” messages, problems with syndication feeds or other issues, try deactivating or removing this plugin.

http://wordpress.org/extend/plugins/adrotate/