CAPTCHA custom engine text field has no accessible name, announced as edit blank

Steps to Reproduce:

1. Log in as admin and go to Administration, CAPTCHA section.
2. Find the captcha engine options ending with the Custom radio option.
3. Next to the Custom radio there is a text field for the custom engine class.
4. Tab to that text field with NVDA running.

Expected Results:

• The text field is announced with its purpose, for example Custom engine, edit, blank 

Actual Results:

• NVDA announces only edit blank: the role (edit box) and the empty value, with no name at all. By comparison, the neighboring captcha answer field is correctly announced as Answer, edit, blank.
• In the page source the field is a bare input with class customEngine and no id, no name and no associated label, while the Custom radio next to it is properly wrapped in a label.