diff -u b/core/lib/Drupal/Core/Cache/Cache.php b/core/lib/Drupal/Core/Cache/Cache.php
--- b/core/lib/Drupal/Core/Cache/Cache.php
+++ b/core/lib/Drupal/Core/Cache/Cache.php
@@ -32,7 +32,7 @@
    * @return string[]
    *   The merged array of cache contexts.
    */
-  public static function mergeContexts($a = array(), $b = array()) {
+  public static function mergeContexts(array $a = array(), array $b = array()) {
     $cache_contexts = array_unique(array_merge($a, $b));
     \Drupal::service('cache_contexts_manager')->validateTokens($cache_contexts);
     sort($cache_contexts);
@@ -58,7 +58,7 @@
    * @return string[]
    *   The merged array of cache tags.
    */
-  public static function mergeTags($a = array(), $b = array()) {
+  public static function mergeTags(array $a = array(), array $b = array()) {
     $cache_tags = array_unique(array_merge($a, $b));
     static::validateTags($cache_tags);
     sort($cache_tags);
diff -u b/core/modules/system/src/Tests/Entity/EntityCacheTagsTestBase.php b/core/modules/system/src/Tests/Entity/EntityCacheTagsTestBase.php
--- b/core/modules/system/src/Tests/Entity/EntityCacheTagsTestBase.php
+++ b/core/modules/system/src/Tests/Entity/EntityCacheTagsTestBase.php
@@ -401,7 +401,7 @@
       $nonempty_entity_listing_cache_tags,
       $page_cache_tags
     );
-    $nonempty_entity_listing_cache_tags = Cache::mergeTags($nonempty_entity_listing_cache_tags);
+
     $this->pass("Test referencing entity.", 'Debug');
     $this->verifyPageCache($referencing_entity_url, 'MISS');