diff -u b/core/modules/system/lib/Drupal/system/Tests/Database/UpdateComplexTest.php b/core/modules/system/lib/Drupal/system/Tests/Database/UpdateComplexTest.php
--- b/core/modules/system/lib/Drupal/system/Tests/Database/UpdateComplexTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Database/UpdateComplexTest.php
@@ -139,11 +139,13 @@
   function testSubSelectUpdate() {
     $subselect = db_select('test', 't');
     $subselect->addExpression('MAX(age) + :increment', 'max_age', array(':increment' => 1));
-    db_update('test_task')
-      ->expression('priority', $subselect)
-      ->execute();
+    $query = db_update('test_task')->expression('priority', $subselect);
+    $string_test = clone $query;
+    $query->execute();
     $results = array_map('intval', db_query('SELECT priority FROM {test_task}')->fetchCol());
     $this->assertIdentical(array(29), array_unique($results));
+    $query_string = preg_replace('/\s+/', ' ', $string_test);
+    $this->assertIdentical('UPDATE {test_task} SET priority= (SELECT MAX(age) + :increment AS max_age FROM {test} t)', $query_string);
   }
 
 }