core/lib/Drupal/Core/Cache/Cache.php | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/core/lib/Drupal/Core/Cache/Cache.php b/core/lib/Drupal/Core/Cache/Cache.php
index e73aee6..587c95d 100644
--- a/core/lib/Drupal/Core/Cache/Cache.php
+++ b/core/lib/Drupal/Core/Cache/Cache.php
@@ -70,16 +70,15 @@ public static function validateContexts(array $context_tokens) {
     // container; it's safe to statically cache this because it cannot change
     // during the request. Initialize the set of valid context tokens with this.
     if (!isset(static::$validContextTokens)) {
-      static::$validContextTokens = \Drupal::getContainer()->getParameter('cache_contexts');
+      static::$validContextTokens = array_flip(\Drupal::getContainer()->getParameter('cache_contexts'));
     }
 
-    $valid = array_flip(static::$validContextTokens);
     foreach ($context_tokens as $context_token) {
       if (!is_string($context_token)) {
         throw new \LogicException('Cache contexts must be strings, ' . gettype($context_token) . ' given.');
       }
 
-      if (isset($valid[$context_token])) {
+      if (isset(static::$validContextTokens[$context_token])) {
         continue;
       }
 
@@ -92,8 +91,8 @@ public static function validateContexts(array $context_tokens) {
       if (strpos($context_id, ':') !== FALSE) {
         $context_id = substr($context_id, 0, strpos($context_id, ':'));
       }
-      if (isset($valid[$context_id])) {
-        static::$validContextTokens[] = $context_token;
+      if (isset(static::$validContextTokens[$context_id])) {
+        static::$validContextTokens[$context_token] = TRUE;
       }
       else {
         throw new \LogicException('"' . $context_id . '" is not a valid cache context ID.');