diff --git a/includes/database/query.inc b/includes/database/query.inc
--- a/includes/database/query.inc
+++ b/includes/database/query.inc
@@ -1898,8 +1898,13 @@ class DatabaseCondition implements QueryConditionInterface, Countable {
   function __clone() {
     $this->changed = TRUE;
     foreach ($this->conditions as $key => $condition) {
-      if ($key !== '#conjunction' && $condition['field'] instanceOf QueryConditionInterface) {
-        $this->conditions[$key]['field'] = clone($condition['field']);
+      if ($key !== '#conjunction') {
+        if ($condition['field'] instanceOf QueryConditionInterface) {
+          $this->conditions[$key]['field'] = clone($condition['field']);
+        }
+        if ($condition['value'] instanceOf SelectQueryInterface) {
+          $this->conditions[$key]['value'] = clone($condition['value']);
+        }
       }
     }
   }
diff --git a/modules/simpletest/tests/database_test.test b/modules/simpletest/tests/database_test.test
--- a/modules/simpletest/tests/database_test.test
+++ b/modules/simpletest/tests/database_test.test
@@ -282,6 +282,45 @@ class DatabaseConnectionTestCase extends DatabaseTestCase {
 }
 
 /**
+ * Test cloning Select queries.
+ */
+class DatabaseSelectCloneTest extends DatabaseTestCase {
+
+  public static function getInfo() {
+    return array(
+      'name' => 'Select tests, cloning',
+      'description' => 'Test cloning Select queries.',
+      'group' => 'Database',
+    );
+  }
+
+  /**
+   * Test that subqueries as value within conditions are cloned properly.
+   */
+  function testSelectConditionSubQueryCloning() {
+    $subquery = db_select('test', 't');
+    $subquery->addField('t', 'id', 'id');
+    $subquery->condition('age', 28, '<');
+
+    $query = db_select('test', 't');
+    $query->addField('t', 'name', 'name');
+    $query->condition('id', $subquery, 'IN');
+
+    $clone = clone $query;
+    // Cloned query should not be altered by the following modification
+    // happening on original query.
+    $subquery->condition('age', 25, '>');
+
+    $clone_result = $clone->countQuery()->execute()->fetchField();
+    $query_result = $query->countQuery()->execute()->fetchField();
+
+    // Make sure the cloned query has not been modified
+    $this->assertEqual(3, $clone_result, 'The cloned query returns the expected number of rows');
+    $this->assertEqual(2, $query_result, 'The query returns the expected number of rows');
+  }
+}
+
+/**
  * Test fetch actions, part 1.
  *
  * We get timeout errors if we try to run too many tests at once.