Refine form validation.

access_code.module

@@ -129,7 +129,7 @@ function access_code_user_profile_form_validate($form, &$form_state) {
     form_set_error('user_access_code', t('The access code can not be longer than 20 characters.'));
   }
 
-  if (drupal_strlen($form_state['values']['access_code']) < 4) {
+  if (!empty($form_state['values']['access_code']) && drupal_strlen($form_state['values']['access_code']) < 4) {
     form_set_error('user_access_code', t('Please choose an access code that is at least 4 characters long. A short access code poses security risk.'));
   }