title: wp_update_user crashes my plugin for wordpress
body: <p>I am trying to write a plugin which will update a user field. I can use the update user meta function OK, but when I try the wp_update_user it doesnt work. </p>
  
  <pre><code>wp_update_user($user_ID, 'user_nicename', 'test');
  </code></pre>
  
  <p>That crashes the plugin. 
  Do I need to include something for this function to work?? </p>