I'm having a problem getting this pagination working it always shows 404 error when I click next.

<ul class="m-grid cf">
        <?php
        $paged = (get_query_var('paged')) ? get_query_var('paged') : 1;
        $args = array('post_type' => 'portfolio','posts_per_page'=> 9, 'order' => 'DESC', 'paged' => $paged );
        query_posts( $args );
        if ( have_posts() ) :
            while ( have_posts() ) : the_post();?>
        <li class="mod-third"><a href="<?php the_permalink();?>">
            <figure class="face thumb">
                <?php
                if ( has_post_thumbnail() ) {
                    the_post_thumbnail('portfolio-thumb');
                }
                ?>
            </figure>
            <h3 class="back"><?php the_title();?></h3>
        </a></li>
        <?php endwhile;


        //Pagination
        if(function_exists('my_paginate_links')) {
            my_paginate_links();
        }

        endif;

        wp_reset_postdata();
        wp_reset_query();

        ?>
    </ul>

& here is the pagination function

function my_paginate_links() {
        global $wp_rewrite, $wp_query;
        $wp_query->query_vars['paged'] > 1 ? $current = $wp_query->query_vars['paged'] : $current = 1;
        $pagination = array(
        'base' => @add_query_arg('page','%#%'),
        'format' => '',
        'total' => $wp_query->max_num_pages,
        'current' => $current,
        'prev_text' => __('« Previous page'),
        'next_text' => __('Next page »'),
        'end_size' => 1,
        'mid_size' => 2,
        'show_all' => true,
        'type' => 'list'
        );
        if ( $wp_rewrite->using_permalinks() )
        $pagination['base'] = user_trailingslashit( trailingslashit( remove_query_arg( 's', get_pagenum_link( 1 ) ) ) . 'page/%#%/', 'paged' );
        if ( !empty( $wp_query->query_vars['s'] ) )
        $pagination['add_args'] = array( 's' => get_query_var( 's' ) );
        echo paginate_links( $pagination );
    }

So what's wrong & how can I fix this?

Thanks.