diff --git a/modules/block/block.admin.inc b/modules/block/block.admin.inc
index b6a8919..b56287f 100644
--- a/modules/block/block.admin.inc
+++ b/modules/block/block.admin.inc
@@ -148,9 +148,10 @@ function block_admin_display_form($form, &$form_state, $blocks, $theme, $block_r
      );
     }
   }
-  // Do not allow disabling the main system content block.
-  $form['blocks']['system_main']['region']['#required'] = TRUE;
-
+  if (isset($form['blocks']['system_main'])) {
+    // Do not allow disabling the main system content block.
+    $form['blocks']['system_main']['region']['#required'] = TRUE;
+  }
   $form['actions'] = array(
     '#tree' => FALSE,
     '#type' => 'actions',