How to hide profile2 field label and captcha label in registration form

I've tried using 2 different functions, where both successfully sets the value to 'invisible' with a dsm($form) check. However, the label is still being displayed.

function MYMODULE_form_alter(&$form, &$form_state, $form_id) {  switch ($form_id) {          case 'user_register_form':              $form['profile_users']['field_username']['#title_display'] = 'invisible';              $form['captcha']['#title_display'] = "invisible";   break;  }}

and:

function MYMODULE_form_profile2_edit_PROFILETYPE_form_alter(&$form, &$form_state){     $form['profile_PROFILETYPE']['field_username']['#title_display'] = 'invisible';}

I do not encounter this problem for the username, password and email fields.

Edit: I've also tried:

$form['profile_users']['field_username']['und']['#title'] = '';$form['profile_users']['field_username']['und']['#title_display'] = "invisible";