Coding problem

Can anyone help me fix this chunk of code that I have in my single.php file? I’m no php guru, but I can’t see why it’s causing a blank page to be returned.

$current_category = single_cat_title("", false);
  $category = get_category($current_category);
  $cat_slug = $category->slug;

  if ($cat_slug == "wolves-in-sheeps-clothing" || $cat_slug == "fruit-that-remains") {
    $my_post_type = "chapter";
    $my_category_type = "this booklet";
  }
  else {
    $my_post_type = "post";
    $my_category_type = "this category";
?>
        <div id="post-navigation">
					<div class="nav-previous"><?php previous_post_link('%link', 'Previous ' . $my_post_type . ' in ' .  $my_category_type, TRUE); ?></div>
					<div class="nav-next"><?php next_post_link('%link', 'Next ' . $my_post_type . ' in ' .  $my_category_type, TRUE); ?></div>
				</div>