PHP Fatal error undefined function apply_filter() addthis_social_widget line 477

hi there 😉

i receive the following error ->

[18-Jul-2012 23:17:41] PHP Fatal error: Call to undefined function apply_filter() in /path/to/wordpress/httpdocs/wp-content/plugins/addthis/addthis_social_widget.php on line 477

but i only do receive this error on sites with the addthis sidebar widget enabled.

all other sites with the addthis above or below the articles / posts are fine!

any help would be more than appreciated 😉

thanks
becki

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