diff --git a/core/lib/Drupal/Core/Cache/Context/HeadersCacheContext.php b/core/lib/Drupal/Core/Cache/Context/HeadersCacheContext.php
index 0a9ac0a..7e4d009 100644
--- a/core/lib/Drupal/Core/Cache/Context/HeadersCacheContext.php
+++ b/core/lib/Drupal/Core/Cache/Context/HeadersCacheContext.php
@@ -25,11 +25,29 @@ public static function getLabel() {
    */
   public function getContext($header = NULL) {
     if ($header === NULL) {
-      return $this->requestStack->getCurrentRequest()->headers->all();
+      $headers = $this->requestStack->getCurrentRequest()->headers->all();
+      // Order headers by name to have less cache variations.
+      ksort($headers);
+      $result = NULL;
+      foreach ($headers as $name => $value) {
+        if ($result) {
+          $result .= '&';
+        }
+        else {
+          $result = '';
+        }
+        $result .= $name . '=' . implode(',', $value);
+      }
+      return $result;
     }
-    else {
-      return $this->requestStack->getCurrentRequest()->headers->get($header);
+    elseif ($this->requestStack->getCurrentRequest()->headers->has($header)) {
+      $value = $this->requestStack->getCurrentRequest()->headers->get($header);
+      if ($value !== '') {
+        return $value;
+      }
+      return '?valueless?';
     }
+    return NULL;
   }
 
   /**
diff --git a/core/tests/Drupal/Tests/Core/Cache/Context/HeadersCacheContextTest.php b/core/tests/Drupal/Tests/Core/Cache/Context/HeadersCacheContextTest.php
index a457695..24ecb2b 100644
--- a/core/tests/Drupal/Tests/Core/Cache/Context/HeadersCacheContextTest.php
+++ b/core/tests/Drupal/Tests/Core/Cache/Context/HeadersCacheContextTest.php
@@ -18,20 +18,17 @@ class HeadersCacheContextTest extends UnitTestCase {
    *
    * @dataProvider providerTestGetContext
    */
-  public function testGetContext($header_name, $header_value, $context) {
+  public function testGetContext($headers, $header_name, $context) {
     $request_stack = new RequestStack();
     $request = Request::create('/', 'GET');
-    if ($header_name) {
-      $request->headers->set($header_name, $header_value);
-    }
+    // Request defaults could change, so compare with default values instead of
+    // passed in context value.
+    $request->headers->replace($headers);
     $request_stack->push($request);
     $cache_context = new HeadersCacheContext($request_stack);
-    if (!$header_name) {
-      // Request defaults could change, so compare with default values instead
-      // of passed in context value.
-      $context = $request->headers->all();
-    }
-    $this->assertSame($cache_context->getContext($header_name), $context);
+    $result = $cache_context->getContext($header_name);
+    file_put_contents('/home/andypost/www/core8/d.log', "result:\n" .var_export($result, TRUE) . "\nexpected:\n" . var_export($context, TRUE) . "\n", FILE_APPEND);
+    $this->assertSame($result, $context);
   }
 
   /**
@@ -39,10 +36,18 @@ public function testGetContext($header_name, $header_value, $context) {
    */
   public function providerTestGetContext() {
     return [
-      [NULL, NULL, NULL],
-      ['x-custom-header', NULL, NULL],
-      ['x-custom-header', '123', '123'],
-      ['x-custom-header', ['123'], '123'],
+      [[], NULL, NULL],
+      [[], 'foo', NULL],
+      // 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],
+      // Header value could be an array.
+      [['z' => ['0', '1']], NULL, 'z=0,1'],
+      [['z' => ['0', '1'], 'a' => []], NULL, 'a=&z=0,1'],
     ];
   }