Error with autoresponder

Hello

I am using the Premise landing page plugin and mailchimp for the autoresponder. When I enter my email and name in the autoresponder optin form I get the following error message:

Fatal error: Class 'WDS_Core_Admin_Tab' not found in /home/genuine/public_html/wp-content/plugins/wpmu-dev-seo/wds-files/wds-sitemaps/wds-sitemaps-settings.php on line 225

What could this be?