diff --git a/javascript_libraries.admin.inc b/javascript_libraries.admin.inc
index bb6092d..245f730 100644
--- a/javascript_libraries.admin.inc
+++ b/javascript_libraries.admin.inc
@@ -423,6 +423,7 @@ function javascript_libraries_edit_form($form, &$form_state, $library = array())
   $form['cache_external'] = array(
     '#type' => 'checkbox',
     '#title' => t('Cache script locally'),
+    '#description' => t('This option only takes effect if JavaScript aggregation is enabled. You must verify that the license or terms of service for the script permit local caching and aggregation.'),
     '#default_value' => isset($library['cache']) ? $library['cache'] : FALSE,
     '#access' => $external_access,
     '#states' => array(