diff --git a/core/lib/Drupal/Core/Cache/Context/HeadersCacheContext.php b/core/lib/Drupal/Core/Cache/Context/HeadersCacheContext.php
index 7e4d009..7dd55bf 100644
--- a/core/lib/Drupal/Core/Cache/Context/HeadersCacheContext.php
+++ b/core/lib/Drupal/Core/Cache/Context/HeadersCacheContext.php
@@ -28,14 +28,11 @@ public function getContext($header = NULL) {
       $headers = $this->requestStack->getCurrentRequest()->headers->all();
       // Order headers by name to have less cache variations.
       ksort($headers);
-      $result = NULL;
+      $result = '';
       foreach ($headers as $name => $value) {
         if ($result) {
           $result .= '&';
         }
-        else {
-          $result = '';
-        }
         $result .= $name . '=' . implode(',', $value);
       }
       return $result;
@@ -47,7 +44,7 @@ public function getContext($header = NULL) {
       }
       return '?valueless?';
     }
-    return NULL;
+    return '';
   }
 
   /**
diff --git a/core/tests/Drupal/Tests/Core/Cache/Context/HeadersCacheContextTest.php b/core/tests/Drupal/Tests/Core/Cache/Context/HeadersCacheContextTest.php
index 53c108d..b70f187 100644
--- a/core/tests/Drupal/Tests/Core/Cache/Context/HeadersCacheContextTest.php
+++ b/core/tests/Drupal/Tests/Core/Cache/Context/HeadersCacheContextTest.php
@@ -34,15 +34,15 @@ public function testGetContext($headers, $header_name, $context) {
    */
   public function providerTestGetContext() {
     return [
-      [[], NULL, NULL],
-      [[], 'foo', NULL],
+      [[], NULL, ''],
+      [[], 'foo', ''],
       // Non-empty headers.
       [['llama' => 'rocks', 'alpaca' => '', 'panda' => 'drools', 'z' => '0'], NULL, 'alpaca=&llama=rocks&panda=drools&z=0'],
       [['llama' => 'rocks', 'alpaca' => '', 'panda' => 'drools', 'z' => '0'], 'llama', 'rocks'],
       [['llama' => 'rocks', 'alpaca' => '', 'panda' => 'drools', 'z' => '0'], 'alpaca', '?valueless?'],
       [['llama' => 'rocks', 'alpaca' => '', 'panda' => 'drools', 'z' => '0'], 'panda', 'drools'],
       [['llama' => 'rocks', 'alpaca' => '', 'panda' => 'drools', 'z' => '0'], 'z', '0'],
-      [['llama' => 'rocks', 'alpaca' => '', 'panda' => 'drools', 'z' => '0'], 'chicken', NULL],
+      [['llama' => 'rocks', 'alpaca' => '', 'panda' => 'drools', 'z' => '0'], 'chicken', ''],
       // Header value could be an array.
       [['z' => ['0', '1']], NULL, 'z=0,1'],
       [['z' => ['0', '1'], 'a' => []], NULL, 'a=&z=0,1'],