diff --git a/core/lib/Drupal/Core/Cache/Context/SessionCacheContext.php b/core/lib/Drupal/Core/Cache/Context/SessionCacheContext.php
index c8b1027..1a88cc4 100644
--- a/core/lib/Drupal/Core/Cache/Context/SessionCacheContext.php
+++ b/core/lib/Drupal/Core/Cache/Context/SessionCacheContext.php
@@ -3,13 +3,14 @@
 namespace Drupal\Core\Cache\Context;
 
 use Drupal\Component\Utility\Crypt;
+use Drupal\Core\Cache\CacheableMetadata;
 
 /**
  * Defines the SessionCacheContext service, for "per session" caching.
  *
  * Cache context ID: 'session'.
  */
-class SessionCacheContext extends RequestStackCacheContextBase {
+class SessionCacheContext extends RequestStackCacheContextBase implements CacheContextInterface {
 
   /**
    * {@inheritdoc}
@@ -22,8 +23,18 @@ public static function getLabel() {
    * {@inheritdoc}
    */
   public function getContext() {
-    $sid = $this->requestStack->getCurrentRequest()->getSession()->getId();
-    return Crypt::hashBase64($sid);
+    $request = $this->requestStack->getCurrentRequest();
+    if ($request->hasSession()) {
+      return Crypt::hashBase64($request->getSession()->getId());
+    }
+    return 'none';
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getCacheableMetadata() {
+    return new CacheableMetadata();
   }
 
 }
diff --git a/core/tests/Drupal/Tests/Core/Cache/Context/SessionCacheContextTest.php b/core/tests/Drupal/Tests/Core/Cache/Context/SessionCacheContextTest.php
index 9538bd2..984c4ab 100644
--- a/core/tests/Drupal/Tests/Core/Cache/Context/SessionCacheContextTest.php
+++ b/core/tests/Drupal/Tests/Core/Cache/Context/SessionCacheContextTest.php
@@ -14,6 +14,13 @@
 class SessionCacheContextTest extends UnitTestCase {
 
   /**
+   * The request.
+   *
+   * @var \Symfony\Component\HttpFoundation\Request
+   */
+  protected $request;
+
+  /**
    * The request stack.
    *
    * @var \Symfony\Component\HttpFoundation\RequestStack
@@ -27,36 +34,30 @@ class SessionCacheContextTest extends UnitTestCase {
    */
   protected $session;
 
-  /**
-   * The session cache context.
-   *
-   * @var \Drupal\Core\Cache\Context\SessionCacheContext
-   */
-  protected $cacheContext;
-
   public function setUp() {
-    $request = new Request();
+    $this->request = new Request();
 
     $this->requestStack = new RequestStack();
-    $this->requestStack->push($request);
-
-    $this->session = $this->getMock('\Symfony\Component\HttpFoundation\Session\SessionInterface');
-    $request->setSession($this->session);
+    $this->requestStack->push($this->request);
 
-    $this->cacheContext = new SessionCacheContext($this->requestStack);
+    $this->session = $this->getMockBuilder('\Symfony\Component\HttpFoundation\Session\SessionInterface')
+      ->getMock();
   }
 
   /**
    * @covers ::getContext
    */
   public function testSameContextForSameSession() {
+    $this->request->setSession($this->session);
+    $cache_context = new SessionCacheContext($this->requestStack);
+
     $session_id = 'aSebeZ52bbM6SvADurQP89SFnEpxY6j8';
     $this->session->expects($this->exactly(2))
       ->method('getId')
       ->will($this->returnValue($session_id));
 
-    $context1 = $this->cacheContext->getContext();
-    $context2 = $this->cacheContext->getContext();
+    $context1 = $cache_context->getContext();
+    $context2 = $cache_context->getContext();
     $this->assertSame($context1, $context2);
     $this->assertSame(FALSE, strpos($context1, $session_id), 'Session ID not contained in cache context');
   }
@@ -65,6 +66,9 @@ public function testSameContextForSameSession() {
    * @covers ::getContext
    */
   public function testDifferentContextForDifferentSession() {
+    $this->request->setSession($this->session);
+    $cache_context = new SessionCacheContext($this->requestStack);
+
     $session1_id = 'pjH_8aSoofyCDQiuVYXJcbfyr-CPtkUY';
     $this->session->expects($this->at(0))
       ->method('getId')
@@ -75,12 +79,22 @@ public function testDifferentContextForDifferentSession() {
       ->method('getId')
       ->will($this->returnValue($session2_id));
 
-    $context1 = $this->cacheContext->getContext();
-    $context2 = $this->cacheContext->getContext();
+    $context1 = $cache_context->getContext();
+    $context2 = $cache_context->getContext();
     $this->assertNotEquals($context1, $context2);
 
     $this->assertSame(FALSE, strpos($context1, $session1_id), 'Session ID not contained in cache context');
     $this->assertSame(FALSE, strpos($context2, $session2_id), 'Session ID not contained in cache context');
   }
 
+  /**
+   * @covers ::getContext
+   * @runInSeparateProcess
+   */
+  public function testContextWithoutSessionInRequest() {
+    $cache_context = new SessionCacheContext($this->requestStack);
+
+    $this->assertSame('none', $cache_context->getContext());
+  }
+
 }