[Plugin: WP Keyword Link] Error on line….

Wonder if anyone can help. I installed this plugin and activated, added one keyword and hit submit. since then i have not been able to get this to work. i have un-installed and re-instal.. a couple times but every time i get a blank screen and this code flashes momentarily on screen:

Fatal error: Call to undefined function mb_strimwidth() in ****/wp-content/plugins/rejected-wp-keyword-link-rejected/wp_keywordlink.php on line 138

When activated it does seem to work even with the 1 keyword i added.
I have had to deactivate for now….

anyone can help. Many thanks in advance.

http://wordpress.org/extend/plugins/rejected-wp-keyword-link-rejected/