diff --git a/core/lib/Drupal/Core/Render/Renderer.php b/core/lib/Drupal/Core/Render/Renderer.php
index dcbd026..45d1b98 100644
--- a/core/lib/Drupal/Core/Render/Renderer.php
+++ b/core/lib/Drupal/Core/Render/Renderer.php
@@ -143,7 +143,7 @@ protected function doRender(&$elements, $is_root_call = FALSE) {
       return '';
     }
 
-    if (isset($elements['#render_children']) && empty($elements['#children']) && $elements['#theme'] != 'page') {
+    if (isset($elements['#render_children']) && empty($elements['#children']) && isset($elements['#theme']) && $elements['#theme'] != 'page') {
       $children_keys = Element::children($elements);
       $output = '';
       foreach ($children_keys as $key) {