Captcha doesn't appear as mandatory on registration form, no error message appears when clicking register without Captcha

Steps to reproduce:

1. Access Administer Wiki
2. Click on Other > CAPTCHA
3. Select Type > Text
4. Select one of the Engine
5. Click on the Save button
6. Click on Registration
7. Tick "Require CAPTCHA to register"
8. Click on the Save button
9. Log out of the wiki
10. Click on Drawer > Register
11. Observe the required fields
12. Complete the fields but not the CAPTCHA
13. Click on Register

Expected results: The Captcha appears as mandatory, if not completed the user can see a message after the Register button is clicked.

Actual results: The Captcha is not marked as mandatory. Once the user completes the required marked fields and clicks on Register nothing happens - the user isn't created but there is no message to inform the user that the Captcha is mandatory.