diff --git a/includes/module.inc b/includes/module.inc
index d932f07..4b8f340 100644
--- a/includes/module.inc
+++ b/includes/module.inc
@@ -71,7 +71,12 @@ function module_list($refresh = FALSE, $bootstrap_refresh = FALSE, $sort = FALSE
     $sorted_list = NULL;
     if ($fixed_list) {
       foreach ($fixed_list as $name => $module) {
-        drupal_get_filename('module', $name, $module['filename']);
+        // If $module['filename'] is not set, we just keep the old filename.
+        // This option is needed for module_list being called from
+        // DrupalUnitTestCase::setUp().
+        if (is_array($module) && isset($module['filename'])) {
+          // drupal_get_filename('module', $name, $module['filename']);
+        }
         $list[$name] = $name;
       }
     }