Problem with wp_update_post

Question

I'm trying to write a plugin that creates a page if a box is checked (value 1) and if the post id doesn't already exist. If the post id does already exist, then I want to update the post with whatever changes have been made. I have the creating post (as page) part working just fine. However, when I try to do run the wp_update_post I get this error:

array_merge() [function.array-merge]: Argument #1 is not an array in /home/wpovernight/wpovernight.com/wp-includes/post.php on line 2996

Since the error is concerning post.php, I'm guessing this issue is with array on wp_update_post, but I'm not sure what I'm doing wrong. It all looks fine to me. Here's my code:

//Check if page exists
$page_check_404 = get_page_by_title($page_title_404);
// Checks if box is checked
if ($activate_404_page == 1) {
            $ss_404_page = array(
            'post_title'    => $page_title_404,
            'post_content'  => $ss_404_page_content,
            'post_status'   => 'publish',
            'post_author'   => 1,
            'post_type'     => 'page'
            );
            // If page doesn't exist, create it
            if(!isset($page_check_404->ID)){
            $ss_404_post_id = wp_insert_post( $ss_404_page );
            }
            //if page does exist, edit it
            if(isset($page_check_404->ID)){
                $ss_404_page_update = array(
                'post_title'    => $page_title_404,
                'ID'            => $ss_404_post_id,
                'post_content'  => $ss_404_page_content,
                'post_status'   => 'publish',
                'post_author'   => 1,
                'post_type'     => 'page'
                );
            wp_update_post( $ss_404_page_update );
            }   
}