diff -u b/core/lib/Drupal/Core/Template/TwigTemplate.php b/core/lib/Drupal/Core/Template/TwigTemplate.php
--- b/core/lib/Drupal/Core/Template/TwigTemplate.php
+++ b/core/lib/Drupal/Core/Template/TwigTemplate.php
@@ -43,7 +43,7 @@
   {
     // Optimized version. NULL is a valid value for $context[$item], we only
     // want to error if it hasn't been defined at all.
-    if (!isset($context[$item]) && !is_null($context[$item])) {
+    if (!isset($context[$item]) && !array_key_exists($item, $context)) {
       // We don't want to throw an exception, but issue a warning instead.
       // This is the easiest way to do so.
       // @todo Decide based on prod vs. dev setting