Copying wrapped entities fails if entity has not been fully loaded

The following code to copy fields from one entity to another fails to do anything unless the commented-out line is restored:

  $node1 = entity_metadata_wrapper('node', $nid1);
  $node2 = entity_metadata_wrapper('node', $nid2);
  foreach ($node1->{$field_name} as $delta => $field) {
    // Calling a function such as getIdentifier() forces the field entity to be fully loaded -- making code work
    // $field->getIdentifier();
    $node2->{$field_name}[] = $field;
  }

A long afternoon of debugging revealed that the cause of the problem is that when EntityDrupalWrapper::setEntity() is called for a wrapped entity, it never checks that the source entity has been fully loaded -- it copies $data->id even though $data->id has never been set.