Drupal 6 form textfield #default_value not working

Surprisingly textfield #default_value is not working in views exposed form. I have enabled search field exposed form and using hook_form_alter I am setting its #default_value property. But while rendering it doesn't show default value. However I found that inside views_handler_filter_search.inc when I change form type textfield to textarea as shown below:

function exposed_form(&$form, &$form_state) {
    if (isset($this->options['expose']['identifier'])) {
      $key = $this->options['expose']['identifier'];
      $form[$key] = array(