diff --git a/hierarchical_select.module b/hierarchical_select.module
index 8540ce2..f73cd22 100644
--- a/hierarchical_select.module
+++ b/hierarchical_select.module
@@ -923,7 +923,9 @@ function _hierarchical_select_validate(&$element, &$form_state) {
 
   // We have to check again for errors. This line is taken litterally from
   // form.inc, so it works in an identical way.
-  if ($element['#required'] && (!count($element['#value']) || (is_string($element['#value']) && strlen(trim($element['#value'])) == 0))) {
+  if ($element['#required'] &&
+    (!isset($form_state['submit_handlers'][0]) || $form_state['submit_handlers'][0] !== 'hierarchical_select_ajax_update_submit') &&
+    (!count($element['#value']) || (is_string($element['#value']) && strlen(trim($element['#value'])) == 0))) {
     form_error($element, t('!name field is required.', array('!name' => $element['#title'])));
     _hierarchical_select_form_set_error_class($element);
   }