If a Mailchimp List has no Groups, then collectMailchimp() gives a warning when it assumes there are some.
Notice: Undefined property: stdClass::$interests in CRM_Mailchimp_Sync->collectMailchimp() (line 158 of path/to/civicrm/sites/default/files/civicrm/ext/uk.co.vedaconsulting.mailchimp/CRM/Mailchimp/Sync.php).
If a Mailchimp List has no Groups, then
collectMailchimp()
gives a warning when it assumes there are some.