EntityMetadataWrapperException: Unknown data property field_comment_ref. in EntityStructureWrapper->getPropertyInfo()

Using the following custom module code:

function message_notify_send_module_comment_insert($comment) {
  $message = message_create('tasktracker_subscription_notification_email', array('uid' => $comment->uid));
  $wrapper = entity_metadata_wrapper('message', $message);
  $wrapper->field_comment_ref->set($comment);
  message_subscribe_send_message('comment', $comment, $message);
}

I get the following error: EntityMetadataWrapperException: Unknown data property field_comment_ref. in EntityStructureWrapper->getPropertyInfo()

When I comment out $wrapper->field_comment_ref->set($comment);, the notification is sent (via e-mail), but no tokens used in the message type are replaced.

Any ideas?