json_encode() errors in log file

One or our clients error log file is littered with heaps of these errors from this plugin.

PHP Warning: json_encode(): Invalid UTF-8 sequence in argument in /home/sites/**SITE REDACTED**/public_html/wp-content/plugins/smart-manager-for-wp-e-commerce/sm/json38.php on line 862, referer: http://www.**SITE REDACTED**/wp-admin/edit.php?post_type=wpsc-product&page=smart-manager-wpsc

Can yoyu look into this please?

Thanks,
Wil.

http://ww.wp.xz.cn/extend/plugins/smart-manager-for-wp-e-commerce/