diff --git a/core/lib/Drupal/Core/Cache/Context/SessionCacheContext.php b/core/lib/Drupal/Core/Cache/Context/SessionCacheContext.php
index bccd0f01a4..40a48f87f9 100644
--- a/core/lib/Drupal/Core/Cache/Context/SessionCacheContext.php
+++ b/core/lib/Drupal/Core/Cache/Context/SessionCacheContext.php
@@ -24,7 +24,11 @@ public static function getLabel() {
   public function getContext() {
     $request = $this->requestStack->getCurrentRequest();
     if ($request->hasSession()) {
-      return Crypt::hashBase64($request->getSession()->getId());
+      $id = $request->getSession()->getId();
+      if (!$id) {
+        throw new Exception('No session ID');
+      }
+      return Crypt::hashBase64($id);
     }
     return 'none';
   }