Website Screenshot Capture

The plugin won’t generate a thumbnail so I switched the WP-config debug ON, any idea with this error:

Notice: Undefined variable: thumbnail_link in /hermes/XXXXXXX/XXXX/XXXXXXXXXXX/XXXXXXXX/wp-content/plugins/wlt_website_thumbnails/class/class_screenshot.php on line 85

The ‘visit website link’ is working (it uses the custom key “url”),so i wonder why it won’t generate a thumbnail. It runs PHP5.3. Thank you.