Mailchimp lists module 7.x-2.10 throws "undefined index" error on site logout

I upgraded Mailchimp from v7.x-2.9 to v7.x-2.10 today in response to an available update notification. I installed and activated the modules (Mailchimp and Mailchimp Lists) as per instructions. The functionality works fine for my limited configuration (i.e., just using Mailchimp Lists to let users subscribe to my newsletter). However, when a user logs out of the system, the following error is thrown:

Notice: Undefined index: default_register_form_optin in mailchimp_lists_auth_newsletter_form() (line 467 of C:\vhosts\anuconch\sites\all\modules\mailchimp\modules\mailchimp_lists\mailchimp_lists.module).

I uninstalled v7.x-2.10 and reverted back to v7.x-2.9 and the error goes away. This behavior is repeatable on my system which is configured as follows:

OS: Win XP Pro SP3
Drupal Version: 7.19
PHP Version: 5.2.17
MySQL Version: 5.5.28
Mailchimp Libraries Version: 1.3

This is not a show-stopper for me since (a) the error doesn't crash my site, (b) my site is in development, and (c) and I can always use Mailchimp v7.x-2.9 anyway.

Thanks in advance for your attention.