I was able to do a quick fix to get mine working again in 3.7. Here is the code I updated in the /lib/search.php file:
if (count($author_in) > 0) {
$authors = implode(',', $author_in);
if ( $authors !== '' ) {
$query_restrictions .= " AND doc IN (SELECT DISTINCT(ID) FROM $wpdb->posts
WHERE post_author IN ($authors))";
}
}
Basically I had to add in a check to see if $authors was an empty string. Looks like it is being set to an array of just a single empty string in 3.7 which makes the count validation true but then breaks the SQL query.
EDIT: Forgot to mention it’s on line 390 in that file