diff --git modules/update/tests/update_test.module modules/update/tests/update_test.module
index fb7d3ab..1d0bd61 100644
--- modules/update/tests/update_test.module
+++ modules/update/tests/update_test.module
@@ -13,6 +13,12 @@ function update_test_menu() {
     'access callback' => TRUE,
     'type' => MENU_CALLBACK,
   );
+  $items['503-error'] = array(
+    'title' => t('503 Service unavailable'),
+    'page callback' => 'update_callback_service_unavailable',
+    'access callback' => TRUE,
+    'type' => MENU_CALLBACK,
+  );
 
   return $items;
 }
@@ -149,3 +155,11 @@ class UpdateTestFileTransfer {
     return $form;
   }
 }
+
+/**
+ * Return an Error 503 (Service unavailable) page.
+ */
+function update_callback_service_unavailable() {
+  drupal_add_http_header('Status', '503 Service unavailable');
+  print "503 Service Temporarily Unavailable";
+}
diff --git modules/update/update.fetch.inc modules/update/update.fetch.inc
index b3041cb..3fafd55 100644
--- modules/update/update.fetch.inc
+++ modules/update/update.fetch.inc
@@ -144,7 +144,7 @@ function _update_process_fetch_task($project) {
 
   if (empty($fail[$fetch_url_base]) || $fail[$fetch_url_base] < $max_fetch_attempts) {
     $xml = drupal_http_request($url);
-    if (isset($xml->data)) {
+    if (!isset($xml->error) && isset($xml->data)) {
       $data = $xml->data;
     }
   }
diff --git modules/update/update.test modules/update/update.test
index a1252dc..e4de648 100644
--- modules/update/update.test
+++ modules/update/update.test
@@ -31,9 +31,9 @@ class UpdateTestHelper extends DrupalWebTestCase {
    *
    * @see update_test_mock_page()
    */
-  protected function refreshUpdateStatus($xml_map) {
+  protected function refreshUpdateStatus($xml_map, $url = 'update-test') {
     // Tell update module to fetch from the URL provided by update_test module.
-    variable_set('update_fetch_url', url('update-test', array('absolute' => TRUE)));
+    variable_set('update_fetch_url', url($url, array('absolute' => TRUE)));
     // Save the map for update_test_mock_page() to use.
     variable_set('update_test_xml_map', $xml_map);
     // Manually check the update status.
@@ -201,6 +201,16 @@ class UpdateCoreTestCase extends UpdateTestHelper {
     $this->assertNoText(t('There is a security update available for your version of Drupal.'));
   }
 
+  /**
+   * Tests the update module when the update server returns 503 (Service unavailable) errors.
+   */
+  function testServiceUnavailable() {
+    $this->refreshUpdateStatus(array(), '503-error');
+    // Ensure that no "Warning: SimpleXMLElement..." parse errors are found.
+    $this->assertNoText('SimpleXMLElement');
+    $this->assertUniqueText(t('Failed to get available update data for one project.'));
+  }
+
   protected function setSystemInfo7_0() {
     $setting = array(
       '#all' => array(