我使用下面的代码应该呈现下一个和上一个帖子缩略图，但它在自定义帖子类型的单页中显示当前帖子缩略图

<?php 

$prevpost = get_previous_post(true);
$prevThumbnail = get_the_post_thumbnail($prevpost->ID, array(80,80) );

next_post_link(\'%link\', \'<span>\' . $prevThumbnail . \'</span>%title\'); 

$nextpost = get_next_post(true);
$nextThumbnail = get_the_post_thumbnail($nextpost->ID, array(80,80) );

previous_post_link(\'%link\', \'<span>\' . $nextThumbnail . \'</span>%title\');           

?>