Block visibility set to specific pages in custom region breaks site in Zen sub-theme

I have a Zen sub-theme with a custom region. The page.tpl is modified to display the region. Everything works as expected until I try to designate a specific page for the custom block to appear on. Then the site content disappears, and the only way to fix it is to edit the visibiliyt field in the blocks table.

Anyone have an idea what could cause this?

Here is my code as it appears in the page.tpl:

  <?php if ($lownavigation): ?>
           <div id="lower_nav"><?php print $lownavigation; ?></div>
            <?php endif; ?>