Call to undefined function mb_list_encodings()

This is a great plugin but I Keep getting this error when trying to run a particular campaign…..

Fatal error: Call to undefined function mb_list_encodings() in /home/*****/public_html/wp-content/plugins/wp-rss-poster/include/tools.php on line 186

I only seem to get this error on a particular feed as it works fine on another without issue.

Works…
http://feeds.feedburner.com/XboxScene-Xbox360-Channel

Doesnt work (get above error)…
http://www.360crunch.net/forum/external.php?type=RSS2&forumids=54

Please can you look into and resolve error please.

Regards
ATHEiST

http://wordpress.org/extend/plugins/wp-rss-poster/