diff --git a/mailchimp.module b/mailchimp.module
index 300f322..ca2b774 100644
--- a/mailchimp.module
+++ b/mailchimp.module
@@ -443,7 +443,7 @@ function mailchimp_addto_queue($function, $args) {
  * @return bool
  *   Success or failure.
  */
-function mailchimp_update_member($list_id, $email, $merge_vars, $format = '', $replace_interests = TRUE) {
+function mailchimp_update_member($list_id, $email, $merge_vars, $format = 'html', $replace_interests = TRUE) {
   if (variable_get('mailchimp_cron', FALSE)) {
     $args = array(
       'list_id' => $list_id,