Item of a list is not defined in an 'if' statement

This code should warn me if an element in the second list is already in the first, after having put the first in lower case.

current_users = ['id_1','id_2','id_3', 'ID_4', 'id_5']
current_users_case = [current_user_case.lower() for current_user_case in current_users]

new_users = ['id_5','id_4','id_7', 'id_8', 'id_9']

for new_user in new_users:
  if new_user == current_user_case:
    print("Sorry, ID already taken")
  else:
    print("ID available")

I get this error message:

Traceback (most recent call last):
  File "main.py", line 2, in <module>
    from c5n9ss import *
  File "/home/runner/C5py/c5n9ss.py", line 11, in <module>
    if new_user == current_user_case:
NameError: name 'current_user_case' is not defined

But if I test the first two lines in the Python shell, I get the lowered list correctly.

I don't understand the error I get.