Clicking "Add another item" two times creates undefined solution

I've tested this with both an integer and a date field. After adding two or more items, answering the CAPTCHA produces an incorrect answer on the first attempt, CAPTCHA then reloads and then works as expected. To reproduce, add a field with unlimited number of entries, add a CAPTCHA test (math in my case) and allow anonymous users to edit/add the content type. Other bits:

- CAPTCHA 6.x-2.3, Drupal 6.20, PHP 5.2.10-2ubuntu6, jQuery UI 1.7.3, jQuery Update 1.3.2
- Caching Mode: Normal
- Prior to hitting Add Another, I have answered the CAPTCHA question with no change in outcome
- Associated error message indicates the solution was "undefined" when in fact it was "2":

car_node_form post blocked by CAPTCHA module: challenge "Math" (by module "captcha"), user answered "2", but the solution was "undefined".