WP-PageNavi help

I have edited WP-PageNavi to use lists for the pages. For the previous and next links I have the following code:

next_posts_link('<li>'.$pagenavi_options['next_text'], $max_page.'</li>');

This wraps the link in <li></li>

However it doesn’t work on:

previous_posts_link('<li>'.$pagenavi_options['prev_text'].'</li>');

The list tags don’t wrap around the link! Any ideas why?