diff --git a/includes/common.inc b/includes/common.inc
index f6171cf..585bb7c 100644
--- a/includes/common.inc
+++ b/includes/common.inc
@@ -3878,7 +3878,15 @@ function drupal_html_id($id) {
       // requested id. $_POST['ajax_html_ids'] contains the ids as they were
       // returned by this function, potentially with the appended counter, so
       // we parse that to reconstruct the $seen_ids array.
-      foreach ($_POST['ajax_html_ids'] as $seen_id) {
+      if (is_array($_POST['ajax_html_ids'])) {
+        $ajax_html_ids = $_POST['ajax_html_ids'];
+      }
+      else {
+        // For some reason (http://drupal.org/node/1575060) jquery.form.js can send string instead of array.
+        // So we need to convert recieved data into an array to keep the logic.
+        $ajax_html_ids = explode(',', $_POST['ajax_html_ids']);
+      }
+      foreach ($ajax_html_ids as $seen_id) {
         // We rely on '--' being used solely for separating a base id from the
         // counter, which this function ensures when returning an id.
         $parts = explode('--', $seen_id, 2);
diff --git a/modules/simpletest/drupal_web_test_case.php b/modules/simpletest/drupal_web_test_case.php
index 694880b..5ab1c14 100644
--- a/modules/simpletest/drupal_web_test_case.php
+++ b/modules/simpletest/drupal_web_test_case.php
@@ -2154,9 +2154,12 @@ class DrupalWebTestCase extends DrupalTestCase {
         $extra_post .= '&' . urlencode($key) . '=' . urlencode($value);
       }
     }
+    $ajax_html_ids = array();
     foreach ($this->xpath('//*[@id]') as $element) {
-      $id = (string) $element['id'];
-      $extra_post .= '&' . urlencode('ajax_html_ids[]') . '=' . urlencode($id);
+      $ajax_html_ids[] = (string) $element['id'];
+    }
+    if (!empty($ajax_html_ids)) {
+      $extra_post .= '&' . urlencode('ajax_html_ids') . '=' . urlencode(implode(',', $ajax_html_ids));
     }
     if (isset($drupal_settings['ajaxPageState'])) {
       $extra_post .= '&' . urlencode('ajax_page_state[theme]') . '=' . urlencode($drupal_settings['ajaxPageState']['theme']);