wp_dropdown_users not showing the list of authors

Hi all,
sorry for the silly question but I’m trying to create a custom form from scratch and in the front end (a simple wp page) I’m putting something like this:

<form action="../process.php" method="post" name="myForm">
Créancier <input id="creancier" type="list" name="creancier" />
<br>
Débiteur <input id="debiteur" type="text" name="debiteur" />
<br>
Noix (quantité) <input id="noix" type="number" name="noix" />
<br>
<input type="submit" value="Submit" /></form>

I see the fields, no problems. But I’d like to include in the form a dropdown list of authors so I pasted this code:

<li id="users">
 <h2><?php _e('users:'); ?></h2>
   <form action="<?php bloginfo('url'); ?>" method="get">
   <?php
     $args = array(
         'exclude'                 => '1',
         'name'                    => 'authors',
         'who'                     => 'authors'
    );
    wp_dropdown_users($args);
   ?>
   <input type="submit" name="submit" value="view" />
   </form>
</li>

The problem is that I see only the bullet and no dropdown list… I tried to include the second piece of code in the first <form> with no result.
Many thanks in advance.