在WordPress中get_categories child_of空数组

I'm attempting to generate a list of categories based on a particular ID in WordPress. The ID is static so here is my code.

<?php $categories =  get_categories('child_of=17'); 
 print_r($categories);      ?>

This just prints out array()

I have made sure that the correct ID is 17, and that there are categories whose parent is the category with ID 17. I'm just wondering does the location of this code matter?

I have it in my own custom sidebar.php file called sidebar-blog.php.

I completely forgot that the categories need to have posts assigned to them before becoming active. My mistake. Theres nothing wrong with that code. Its working now.