diff --git a/includes/common.inc b/includes/common.inc
index c23c80b..2fbb820 100644
--- a/includes/common.inc
+++ b/includes/common.inc
@@ -3900,7 +3900,7 @@ function drupal_clean_css_identifier($identifier, $filter = array(' ' => '-', '_
   }
   $allow_css_double_underscores = &$drupal_static_fast['allow_css_double_underscores'];
   if (!isset($allow_css_double_underscores)) {
-    $allow_css_double_underscores = variable_get('allow_css_double_underscores');
+    $allow_css_double_underscores = variable_get('allow_css_double_underscores', FALSE);
   }
 
   // Preserve BEM-style double-underscores depending on custom setting.
diff --git a/modules/simpletest/tests/common.test b/modules/simpletest/tests/common.test
index 61f4912..7aca10c 100644
--- a/modules/simpletest/tests/common.test
+++ b/modules/simpletest/tests/common.test
@@ -956,12 +956,22 @@ class DrupalHTMLIdentifierTestCase extends DrupalUnitTestCase {
     // Mock a variable_set of allow_css_double_underscores to TRUE.
     $original = isset($GLOBALS['conf']['allow_css_double_underscores']) ? $GLOBALS['conf']['allow_css_double_underscores'] : NULL;
     $GLOBALS['conf']['allow_css_double_underscores'] = TRUE;
+    $this->resetDrupalCleanCSSIdentifierCache();
 
     // Now, verify that double underscores are preserved in the identifier.
     $this->assertIdentical(drupal_clean_css_identifier($identifier), $identifier, 'Verify double underscores are preserved.');
 
-    // Revert the overridden value above.
+    // To avoid affecting other test cases, revert the overridden value above
+    // and reset the cache.
     $GLOBALS['conf']['allow_css_double_underscores'] = $original;
+    $this->resetDrupalCleanCSSIdentifierCache();
+  }
+
+  /**
+   * reset the cache in drupal_clean_css_identifier().
+   */
+  function resetDrupalCleanCSSIdentifierCache() {
+    drupal_static_reset('drupal_clean_css_identifier:allow_css_double_underscores');
   }
 
   /**