Using entity metadata wrapper with custom entity type

Question

I've created custom entity. In addition I want to add user reference field to my entity:

if (! field_info_field(MY_FIELD_NAME)) {
  // Create the field.
   $field = array(
    'field_name' => MY_FIELD_NAME,
    'type' => 'entityreference',
    'cardinality' => FIELD_CARDINALITY_UNLIMITED,
    'settings' => array(
        'target_type' => 'user',
        'handler' => 'base',
        'handler_settings' => array(
            'target_bundles' => array()
        ),
    ),
    'bundles' => array (
        MY_ENTITY_NAME =>
            array (
                0 => MY_ENTITY_BUNDLE_NAME,
            ),
    ),
);
field_create_field($field);

    // Create the instance.
    $instance = array(
        'field_name' => MY_FIELD_NAME,
        'entity_type' => MY_ENTITY_NAME,
        'bundle' => MY_ENTITY_BUNDLE_NAME,
        'label' => '',
        'description' => '',
        'required' => TRUE,
        'widget' => array(),
        'display' => array(
            'default' => array(
                'label' => 'hidden',
            ),
        ),
    );

    field_create_instance($instance);
}

In form submit I want to add users references to this field:

   foreach($result as $recipient) {
        $wrapper->{MY_FIELD_NAME}[] = $recipient->uid;
    }

I gets:

Fatal error: Cannot use object of type EntityDrupalWrapper as array

What I am missing?