Ticket #16850: test-16850.php

<?php

add_action( 'init', function() {
        $q = new WP_Query( array( 'post_parent' => array( '', 1, 2, 'billy tables' ) ) );

        var_dump($q->request);

        $q = new WP_Query( array( 'post_parent' => 'billy tables' ) );

        var_dump($q->request);

        $q = new WP_Query( array( 'post_parent' => 1 ) );

        var_dump($q->request);
} );