--- simpletest.module	2009-12-14 15:27:11.000000000 -0800
+++ simpletest-692358.module	2010-08-13 10:33:19.408787486 -0700
@@ -208,7 +208,9 @@
   // Drupal 6.
   require_once drupal_get_path('module', 'simpletest') . '/drupal_web_test_case.php';
   $classes = simpletest_test_get_all_classes();
-  require_once $classes[$test_class]['file'];
+  if(!empty($classes[$test_class]['file'])){
+    require_once $classes[$test_class]['file'];
+  }
 
   $test = new $test_class($test_id);
   $test->run();