diff --git a/core/modules/system/src/Tests/Theme/EngineTwigTest.php b/core/modules/system/src/Tests/Theme/EngineTwigTest.php
index 02139ab..c6139ed 100644
--- a/core/modules/system/src/Tests/Theme/EngineTwigTest.php
+++ b/core/modules/system/src/Tests/Theme/EngineTwigTest.php
@@ -103,7 +103,7 @@ public function testTwigUrlToString() {
   }
 
   /**
-   * Tests the file_url Twig functions.
+   * Tests the automatic/magic calling of toString() on objects, if exists.
    */
   public function testTwigFileUrls() {
     $this->drupalGet('/twig-theme-test/file-url');
diff --git a/core/themes/engines/twig/twig.engine b/core/themes/engines/twig/twig.engine
index 46646d9..db71af5 100644
--- a/core/themes/engines/twig/twig.engine
+++ b/core/themes/engines/twig/twig.engine
@@ -150,6 +150,9 @@ function twig_render_var($arg) {
     if (method_exists($arg, '__toString')) {
       return (string) $arg;
     }
+    // You can't throw exceptions in the magic PHP __toString methods, see
+    // http://php.net/manual/en/language.oop5.magic.php#object.tostring so
+    // we also support a toString method.
     elseif (method_exists($arg, 'toString')) {
       return $arg->toString();
     }
@@ -240,6 +243,9 @@ function twig_drupal_escape_filter(\Twig_Environment $env, $string, $strategy =
     if (method_exists($string, '__toString')) {
       $return = (string) $string;
     }
+    // You can't throw exceptions in the magic PHP __toString methods, see
+    // http://php.net/manual/en/language.oop5.magic.php#object.tostring so
+    // we also support a toString method.
     elseif (method_exists($string, 'toString')) {
       $return = $string->toString();
     }