Hello All,
Still moving along here with my zc1.5.8 on php8.2.3. I just found out that on the product info page adding zero quantity to cart breaks the site. I've tried on another far less modified install and have the same result. I found another post that leaned toward USU. I disabled USU but still the same result.

On the product info page if I change qty to 0 (zero) and click add to cart I get a white screen. Error log here

Code:
[04-Mar-2023 02:24:47 UTC] Request URI: /the-category-c-3/the-product-name-p-2120.html?action=add_product, IP address: 111.222.333.444
--> PHP Fatal error: Uncaught Error: Undefined constant "TOPMOST_CATEGORY_PARENT_ID" in /home/acct123/dir.domain.com/includes/functions/functions_categories.php:186
Stack trace:
#0 /home/acct123/dir.domain.com/includes/functions/functions_categories.php(639): zen_get_parent_categories(Array, '3')
#1 /home/acct123/dir.domain.com/includes/classes/shopping_cart.php(1913): zen_get_generated_category_path_rev('3')
#2 /home/acct123/dir.domain.com/includes/main_cart_actions.php(47): shoppingCart->actionAddProduct('shopping_cart', Array)
#3 /home/acct123/dir.domain.com/includes/init_includes/init_cart_handler.php(42): require('/home/acct123...')
#4 /home/acct123/dir.domain.com/includes/autoload_func.php(40): require_once('/home/acct123...')
#5 /home/acct123/dir.domain.com/includes/application_top.php(237): require('/home/acct123...')
#6 /home/acct123/dir.domain.com/index.php(25): require('/home/acct123...')
#7 {main}
  thrown in /home/acct123/dir.domain.com/includes/functions/functions_categories.php on line 186.
Any ideas?

Thank you,
John