diff -u b/core/lib/Drupal/Core/Template/TwigExtension.php b/core/lib/Drupal/Core/Template/TwigExtension.php
--- b/core/lib/Drupal/Core/Template/TwigExtension.php
+++ b/core/lib/Drupal/Core/Template/TwigExtension.php
@@ -374,7 +374,7 @@
    *   The escaped, rendered output, or NULL if there is no valid output.
    *
    * @throws \Exception
-   *   When an object is passed that has no __toString() method.
+   *   When passed $string is an object that has no __toString() method.
    */
   public function escapePlaceholder($env, $string) {
     return '<em class="placeholder">' . $this->escapeFilter($env, $string) . '</em>';
@@ -404,7 +404,7 @@
    *   The escaped, rendered output, or NULL if there is no valid output.
    *
    * @throws \Exception
-   *   When an object is passed that has no __toString() method.
+   *   When passed $arg is an object that has no __toString() method.
    *
    * @todo Refactor this to keep it in sync with theme_render_and_autoescape()
    *   in https://www.drupal.org/node/2575065
@@ -490,7 +490,7 @@
    *   The rendered output or an Twig_Markup object.
    *
    * @throws \Exception
-   *   When an object is passed that has no __toString() method.
+   *   When passed $arg is an object that has no __toString() method.
    *
    * @see render
    * @see TwigNodeVisitor