WordPress register_nav_menu

// In theme_folder/functions.php
<?php
function bt_register_custom_menus() {
  register_nav_menu('header_left', __('Header Left'));
  register_nav_menu('header_right', __('Header Right'));
  register_nav_menu('home_left', __('Homepage Left'));
  register_nav_menu('home_right', __('Homepage Right'));
}
add_action('init', 'bt_register_custom_menus');
?>

// In theme_folder/header.php
<?php wp_nav_menu( array('menu' => 'header_left' )); ?>
<?php wp_nav_menu( array('menu' => 'header_right' )); ?>

When the page is displayed, the left and right menus do not get found.  Instead it reverts to the default menu (lowest id).