[Plugin: WP Super Cache] PHP Fatal error: Call to undefined function get_supercache_dir()

Anybody know how to fix this? It’s calling to this line of code
$path = get_supercache_dir() . preg_replace( '/:.*$/', '', $_GET[ 'path' ] );

It seems to relate to when I try to delete the cache on a single page.

http://ww.wp.xz.cn/extend/plugins/wp-super-cache/