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
@@ -137,15 +137,23 @@
    * Test UPDATE with a subselect value.
    */
   function testSubSelectUpdate() {
-    $subselect = db_select('test', 't');
-    $subselect->addExpression('MAX(age) + :increment', 'max_age', array(':increment' => 1));
-    $query = db_update('test_task')->expression('priority', $subselect);
-    $string_test = clone $query;
+    $subselect = db_select('test_task', 't');
+    $subselect->addExpression('MAX(priority) + :increment', 'max_priority', array(':increment' => 30));
+    // Clone this to make sure we are running a different query when
+    // asserting.
+    $select = clone $subselect;
+    $query = db_update('test')
+      ->expression('age', $subselect)
+      ->condition('name', 'Ringo');
+    // Save the query for a __toString test later.
+    $string_test = $query;
     $query->execute();
-    $results = array_map('intval', db_query('SELECT priority FROM {test_task}')->fetchCol());
-    $this->assertIdentical(array(29), array_unique($results));
+    $after_age = db_query('SELECT age FROM {test} WHERE name = :name', array(':name' => 'Ringo'))->fetchField();
+    $expected_age = $select->execute()->fetchField();
+    $this->assertEqual($after_age, $expected_age);
+    // Replace whitespace with a single space.
     $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);
+    $this->assertIdentical('UPDATE {test} SET age= (SELECT MAX(priority) + :increment AS max_priority FROM {test_task} t) WHERE (name = :db_condition_placeholder_0)', trim($query_string));
   }
 
 }