diff --git a/modules/simpletest/tests/common.test b/modules/simpletest/tests/common.test
index 437b67c..b6968b5 100644
--- a/modules/simpletest/tests/common.test
+++ b/modules/simpletest/tests/common.test
@@ -781,6 +781,14 @@ class CascadingStylesheetsTestCase extends DrupalWebTestCase {
     $styles = drupal_get_css();
     $this->assert(strpos($styles, $system . '/system.base.css') !== FALSE, t('The overriding CSS file is output.'));
     $this->assert(strpos($styles, $simpletest . '/tests/system.base.css') === FALSE, t('The overridden CSS file is not output.'));
+
+    drupal_add_css($system . '/system.base.css');
+    drupal_add_css($simpletest . '/tests/system.base.css');
+
+    // The dummy stylesheet should be the only one included.
+    $styles = drupal_get_css();
+    $this->assert(strpos($styles, $simpletest . '/tests/system.base.css') !== FALSE, t('The overriding CSS file is output.'));
+    $this->assert(strpos($styles, $system . '/system.base.css') === FALSE, t('The overridden CSS file is not output.'));
   }
 
   /**