WP3.6 introduced a "Strict Standards" notice

After updating to WordPress 3.6, my theme reports a “Strict Standards” notice on each page of the site, even in wp-admin:

Strict Standards: call_user_func_array() expects parameter 1 to be a valid callback, non-static method Facetious_Widget::register() should not be called statically in path\to\wordpress\wp-includes\plugin.php on line 406

Is this something inside the plugin or the way I have implemented the form your plugin creates? I presume it’s something with the plugin core itself, as it appears on pages where I do not even use the form. I use

$args = array( ... );
$form = facetious( $args ); // No echo
// some mods to the function output
echo $form;

to output the Facetious Search form in a certain sidebar template.

I have WP_DEBUG turned on.

https://www.ads-software.com/plugins/facetious/