diff --git a/advagg.module b/advagg.module
index 9451d2f..8d0af84 100644
--- a/advagg.module
+++ b/advagg.module
@@ -535,9 +535,16 @@ function _advagg_process_html(&$variables) {
   if (isset($variables['page']['page_top'])) {
     $variables['page_top'] = drupal_render($variables['page']['page_top']);
   }
+  elseif (!isset($variables['page_top'])) {
+    $variables['page_top'] = '';
+  }
   if (isset($variables['page']['page_bottom'])) {
     $variables['page_bottom'] = drupal_render($variables['page']['page_bottom']);
   }
+  elseif (!isset($variables['page_bottom'])) {
+    $variables['page_bottom'] = '';
+  }
+
   // Place the rendered HTML for the page body into a top level variable.
   if (isset($variables['page']['#children'])) {
     $variables['page'] = $variables['page']['#children'];