drupal_process_form sets $form_state['values'] to empty array

API page: http://api.drupal.org/api/drupal/includes%21form.inc/function/drupal_pro...

I'm writing a migration from a 3rd party CMS and ran into this issue. When trying to update a node, the values passed in using $form_state['values'] were overwritten to an empty array by the first line in this function:

function drupal_process_form($form_id, &$form, &$form_state) {
  $form_state['values'] = array();

After commenting out the line, my API calls were successful, but the UI broke, so I'm assuming somewhere this function is called without $form_state['values'] being set, hence the initial setting. I modified the function to check for the values instead and both API and UI are now working as expected:

function drupal_process_form($form_id, &$form, &$form_state) {
    if (!isset($form_state['values'])) {
      $form_state['values'] = array();
    }