wp_nav_menu only shows first menu

Im having a problem with wp_nav_menu seemingly ignoring the menu parameter – regardless of what I set as the value, it always shows the first menu I have (out of the two). Theme Locations in the admin gui shows two supported menus, both populated (‘Header’ and ‘Sibebar’)

Have the following in my header:

<? wp_nav_menu( array('menu' => 'First Menu' )); ?>

and in my sidebar:

<? wp_nav_menu( array(‘menu’ => ‘Second Menu’ )); ?>

Have the following code in my functions.php to register two menus with my theme:

if ( function_exists( 'register_nav_menus' ) ) {
  	register_nav_menus(
  		array(
  		  'header_menu' => 'First Menu',
  		  'sidebar_menu' => 'Second Menu'
  		)
  	);
}

The code you see above was taken from another post here that the user said worked fine, so Im at a bit of a loss as to what Im doing wrong.