diff --git a/core/lib/Drupal/Component/Utility/SafeMarkup.php b/core/lib/Drupal/Component/Utility/SafeMarkup.php
index 2d5b22a..db7557e 100644
--- a/core/lib/Drupal/Component/Utility/SafeMarkup.php
+++ b/core/lib/Drupal/Component/Utility/SafeMarkup.php
@@ -82,7 +82,7 @@ public static function set($string, $strategy = 'html') {
   /**
    * Checks if a string is safe to output.
    *
-   * @param string $string
+   * @param string|\Drupal\Component\Utility\SafeStringInterface $string
    *   The content to be checked.
    * @param string $strategy
    *   The escaping strategy. See self::set(). Defaults to 'html'.
@@ -91,6 +91,8 @@ public static function set($string, $strategy = 'html') {
    *   TRUE if the string has been marked secure, FALSE otherwise.
    */
   public static function isSafe($string, $strategy = 'html') {
+    // Do the instance of checks first to save unnecessarily casting the object
+    // to a string.
     return $string instanceOf SafeStringInterface || isset(static::$safeStrings[(string) $string][$strategy]) ||
       isset(static::$safeStrings[(string) $string]['all']);
   }
diff --git a/core/lib/Drupal/Component/Utility/SafeStringInterface.php b/core/lib/Drupal/Component/Utility/SafeStringInterface.php
index f078faa..1414a37 100644
--- a/core/lib/Drupal/Component/Utility/SafeStringInterface.php
+++ b/core/lib/Drupal/Component/Utility/SafeStringInterface.php
@@ -8,7 +8,7 @@
 namespace Drupal\Component\Utility;
 
 /**
- * Marks an objects __toString() method as returning safe markup.
+ * Marks an object's __toString() method as returning safe markup.
  */
 interface SafeStringInterface {
 }