mail chimp returns php error on user subscription signup

warning: Invalid argument supplied for foreach() in /hermes/web10/b2808/moo.aveotsd2ucom/drupcomtest/modules/mailchimp-6.x-2.3/mailchimp/mailchimp.module on line 509.

argument on line 509

foreach ($lists as $list) {
if (!empty($list['EMAIL'])) {
$success = FALSE;

// include interest groups
if (isset($list['interest_groups_' . $list['list']->id]) && is_array($list['interest_groups_' . $list['list']->id])) {
foreach($list['interest_groups_' . $list['list']->id] as $key => $group) {
$list['GROUPINGS'][] = array('id' => $key, 'groups' => is_array($group) ? implode(', ', array_filter($group)) : $group);
}
}

as a side note i have had the same error from version 2.4