Pagination on the single.php template

I’m having a bit of a (recent) problem with posts that are split into separate pages using <!–nextpage–>. On each of the split posts, clicking any of the pages (e.g., 4, 3, etc.) simply reloads the first page. The URL bar in the browser (doesn’t matter which browser) at first shows the page requested (e.g., /blogpost/2/), but changes during load and displays the permalink instead (e.g., /blogpost/).

Example here: http://northridgemedia.net/blogs/create/departments/inspiration/how-its-made-recycled-frame-chalkboard/.

Poking around Google results, it looks like most of the problems relate to custom queries, which I’m not using at the moment. I’m using a pretty basic single.php, like below:

<?php the_content('<p>Read the rest of this entry &raquo;</p>'); ?>
			<?php wp_link_pages(array('before' => '<div id="postpagenav"><ul id="ppn"><li><strong>Pages:</strong></li> ', 'after' => '</ul></div>', 'next_or_number' => 'number' , 'link_before' => '<li>' , 'link_after' => '</li>')); ?>

Any thoughts?