Fatal error at activation

When I try to activate the plugin, I get this error:

Fatal error: Uncaught Error: Non-static method Wp_Ecards_Activator::wp_ecards_install() cannot be called statically in /data/web/e14661/html/apps/wordpress-121852/wp-content/plugins/wp-ecards-invites/includes/class-wp-ecards-activator.php:33 Stack trace: #0 /data/web/e14661/html/apps/wordpress-121852/wp-content/plugins/wp-ecards-invites/wp-ecards.php(42): Wp_Ecards_Activator::activate() #1 /data/web/e14661/html/apps/wordpress-121852/wp-includes/class-wp-hook.php(292): activate_wp_ecards() #2 /data/web/e14661/html/apps/wordpress-121852/wp-includes/class-wp-hook.php(316): WP_Hook->apply_filters() #3 /data/web/e14661/html/apps/wordpress-121852/wp-includes/plugin.php(484): WP_Hook->do_action() #4 /data/web/e14661/html/apps/wordpress-121852/wp-admin/plugins.php(193): do_action() #5 {main} thrown in /data/web/e14661/html/apps/wordpress-121852/wp-content/plugins/wp-ecards-invites/includes/class-wp-ecards-activator.php on line 33