WordPress Pagination – max_num_pages = 0

WordPress ninjas… I require some help regarding pagination.

I have the following code :

global $wp_query;
 $paged = (get_query_var('paged')) ? get_query_var('paged') : 1;
 $args = array_merge( $wp_query->query_vars, 
        array( 
                'post_type' => 'attachment',
                'posts_per_page' => 10,
                'paged' => $paged                                       
        )
 );

How come $query->max_num_pages returns 0 ?

And is there a way to alter the original archive query with the $query->max_num_pages ?

[EDIT]

In Addition to the above code I have the following

 $output = '<div class="download-attachments left_content_container"><ul>';

//this does not retrieve anything 
 $query = new WP_Query( $args );

// this does retrieve posts
$attachments = get_posts( $args );

// this block is only used for testing purposes.. 
// ie. it is not being executed because the query does not return anything, Whilst get_posts()    works 



 if( $query->have_posts() ){
         while( $query->have_posts() ){
             $query->the_post();
             echo the_ID();
         }
    }

 foreach ($attachments as $attachment) {
        setup_postdata($attachment);

        //get attachments metadata etc.
        $url = wp_get_attachment_url( $attachment->ID );
        $title = get_the_title( $attachment );
        $caption = $attachment->post_excerpt;
        $size = get_readable_size( filesize(get_attached_file( $attachment->ID )) );
        $icon =  get_icon_for_attachment( $attachment->ID );
        $mime = get_post_mime_type(  $attachment->ID );
        $date_added = $attachment->post_date;
        $filename = ( !$caption == '' ) ? $caption : $title ;
        $description =  ($attachment->post_content != '') ? '<span class="attachment-author"><span class="attachemnt-label">Added by: ' . $attachment->post_content . '</span></span>' : '';
        $output .= '<li class="'.$mime.'">
                    <img class="attachment-icon" src="'.$icon.'" alt="pdf"> 
                    <a href="'. home_url().'/wp-content/plugins/download-attachments/includes/download.php?id='.$attachment->ID.'" class="attachment-link" title="'.$filename.'">'. $filename  .'</a>       '.$description.'
                    <span class="attachment-date"><span class="attachment-label">Date added:  </span>'.$date_added.'</span> 
                    <span class="attachment-size"><span class="attachment-label">Attachment size:  </span>'.$size.'</span></li>' ;


 }
 $output .= '</ul></div>';

 echo $output;


 echo '<div class="paging pull-left">';
        $paged = $wp_query->get(  'paged' );
        if ( ! $paged) {/* do nothing */ ;}
        else { bootstrap_pagination(); var_dump($wp_query->max_num_pages); // returns 0 }
echo '</div>';