diff --git a/modules/block/block.module b/modules/block/block.module
index 7abbe2f19cd..a44b910a4c1 100644
--- a/modules/block/block.module
+++ b/modules/block/block.module
@@ -456,6 +456,22 @@ function block_form_system_performance_settings_alter(&$form, &$form_state) {
     '#disabled' => count(module_implements('node_grants')),
     '#description' => t('Note that block caching is inactive when modules defining content access restrictions are enabled.'),
   );
+  
+  // Check if the "Who's online" block is enabled.
+  $online_block_enabled = db_select('block')
+    ->condition('module', 'user')
+    ->condition('delta', 'online')
+    ->condition('status', 1)
+    ->countQuery()
+    ->execute()
+    ->fetchField();
+  
+  // If the "Who's online" block is enabled, append some descriptive text to
+  // the end of the form description.
+  if ($online_block_enabled) {
+    $form['page_cache']['cache']['#description'] .=  '<p>' . t('When caching is enabled, anonymous user sessions are only saved to the database when needed, so the "Who\'s online" block does not display the number of anonymous users.') . '</p>';
+  }
+  
 }
 
 /**
diff --git a/modules/system/system.admin.inc b/modules/system/system.admin.inc
index eb15ce009fd..481355b4026 100644
--- a/modules/system/system.admin.inc
+++ b/modules/system/system.admin.inc
@@ -1296,18 +1296,6 @@ function system_performance_settings() {
 
   $description = '<p>' . t("The normal cache mode is suitable for most sites and does not cause any side effects. The aggressive cache mode causes Drupal to skip the loading (boot) and unloading (exit) of enabled modules when serving a cached page. This results in an additional performance boost but can cause unwanted side effects.") . '</p>';
 
-  // Check if the "Who's online" block is enabled.
-  $online_block_enabled = db_select('block')
-    ->condition('module', 'user')
-    ->condition('delta', 'online')
-    ->condition('status', 1)
-    ->countQuery()
-    ->execute()
-    ->fetchField();
-  if ($online_block_enabled) {
-    $description .= '<p>' . t("When caching is enabled, anonymous user sessions are only saved to the database when needed, so the \"Who's online\" block does not display the number of anonymous users.") . '</p>';
-  }
-
   $problem_modules = array_unique(array_merge(module_implements('boot'), module_implements('exit')));
   sort($problem_modules);