core/lib/Drupal/Core/Block/BlockBase.php     | 7 ++++---
 core/lib/Drupal/Core/Cache/CacheContexts.php | 8 +++++---
 2 files changed, 9 insertions(+), 6 deletions(-)

diff --git a/core/lib/Drupal/Core/Block/BlockBase.php b/core/lib/Drupal/Core/Block/BlockBase.php
index 8a92e2a..796a4c7 100644
--- a/core/lib/Drupal/Core/Block/BlockBase.php
+++ b/core/lib/Drupal/Core/Block/BlockBase.php
@@ -225,9 +225,10 @@ public function buildConfigurationForm(array $form, FormStateInterface $form_sta
       // choose to modify by: they must always be applied.
       $context_labels = array();
       $all_contexts = \Drupal::service("cache_contexts")->getLabels(TRUE);
-      foreach (array_keys(CacheContexts::parseTokens($this->getRequiredCacheContexts())) as $context) {
-        $context_labels[] = $all_contexts[$context];
-        unset($form['cache']['contexts']['#options'][$context]);
+      foreach (CacheContexts::parseTokens($this->getRequiredCacheContexts()) as $context) {
+        $context_id = $context[0];
+        $context_labels[] = $all_contexts[$context_id];
+        unset($form['cache']['contexts']['#options'][$context_id]);
       }
       $required_context_list = implode(', ', $context_labels);
       $form['cache']['contexts']['#description'] .= ' ' . t('This block is <em>always</em> varied by the following contexts: %required-context-list.', array('%required-context-list' => $required_context_list));
diff --git a/core/lib/Drupal/Core/Cache/CacheContexts.php b/core/lib/Drupal/Core/Cache/CacheContexts.php
index d6146b5..4c0253f 100644
--- a/core/lib/Drupal/Core/Cache/CacheContexts.php
+++ b/core/lib/Drupal/Core/Cache/CacheContexts.php
@@ -137,9 +137,11 @@ protected function getService($context_id) {
    *   An array of cache context tokens.
    *
    * @return array
-   *   An array with the parsed results, with the cache context IDs as keys, and
-   *   the associated parameter as value (for a calculated cache context), or
-   *   NULL if there is no parameter.
+   *   An array with the parsed results, with each result being an array
+   *   containing:
+   *   1. the cache context ID
+   *   2. the associated parameter (for a calculated cache context), or NULL if
+   *      there is no parameter.
    */
   public static function parseTokens(array $context_tokens) {
     $contexts_with_parameters = [];