Custom Post Type and single-posttype template

I found solution. Big Thanks @Scribu Alternative to query_posts for main loop? Now works fine and no more 404 error.

function wpse49295_alter_the_query( $request )
{
    $dummy_query = new WP_Query(); 
    $dummy_query->parse_query( $request );

    if ( $dummy_query->is_single() )
        $request['post_type'] = 'topic';

    return $request;
}
add_filter( 'request', 'wpse49295_alter_the_query' );