diff --git a/core/lib/Drupal/Core/StringTranslation/TranslatableMarkup.php b/core/lib/Drupal/Core/StringTranslation/TranslatableMarkup.php
index 8b4902a..3d42014 100644
--- a/core/lib/Drupal/Core/StringTranslation/TranslatableMarkup.php
+++ b/core/lib/Drupal/Core/StringTranslation/TranslatableMarkup.php
@@ -75,7 +75,9 @@ class TranslatableMarkup extends FormattableMarkup {
    * @see \Drupal\Component\Render\FormattableMarkup::placeholderFormat()
    */
   public function __construct($string, array $arguments = array(), array $options = array(), TranslationInterface $string_translation = NULL) {
-    $this->string = $string;
+    // @todo Decide whether to enforce $string to be a string.
+    //   https://www.drupal.org/node/2570285
+    $this->string = (string) $string;
     $this->arguments = $arguments;
     $this->options = $options;
     $this->stringTranslation = $string_translation;