dimitriskr
--- a/core/lib/Drupal/Core/Cache/CacheableMetadata.php
+++ b/core/lib/Drupal/Core/Cache/CacheableMetadata.php
    else {
      $result->cacheMaxAge = Cache::mergeMaxAges($this->cacheMaxAge, $other->cacheMaxAge);
    }
+    assert($result instanceof CacheableMetadata);
--- a/core/lib/Drupal/Core/Cache/Context/QueryArgsCacheContext.php
+++ b/core/lib/Drupal/Core/Cache/Context/QueryArgsCacheContext.php
      // All arguments requested. Use normalized query string to minimize
      // variations.
      $value = $this->requestStack->getCurrentRequest()->getQueryString();
-      return ($value !== NULL) ? $value : '';
+      return ($value !== NULL) ? (string) $value : '';
    }
    elseif ($this->requestStack->getCurrentRequest()->query->has($query_arg)) {
      $value = $this->requestStack->getCurrentRequest()->query->all()[$query_arg];
      if (is_array($value)) {
-        return http_build_query($value);
+        return http_build_query([$query_arg => $value], '', '&');