Wordpress wp_link_pages show only next/prev link

I used nextpage to separate a post into multiple page and I don't want to show pagination as 1 2 3 Next, I want to to show Prev Next, below is the code I come up with after reading the wp link pages, it is still showing 1 2 3 Next, I don't know what I did wrong, any help will be much appreciated

<?php
wp_link_pages( array( 'before' => '', 'after' => '', 
'previouspagelink'    => '', 'nextpagelink' => '<span>Next post</span>', 
'next_or_number' => 'next' ) ); 
?>