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
@@ -451,7 +451,7 @@
     }
     elseif (is_object($arg) && !($arg instanceof ReadOnlyArrayObject)) {
       if ($arg instanceof RenderableInterface) {
-        $arg = $arg->toRenderable();
+        $arg = new ReadOnlyArrayObject($arg->toRenderable());
       }
       elseif (method_exists($arg, '__toString')) {
         $return = (string) $arg;
@@ -569,7 +569,7 @@
     else if (is_object($arg) && !($arg instanceof ReadOnlyArrayObject)) {
       $this->bubbleArgMetadata($arg);
       if ($arg instanceof RenderableInterface) {
-        $arg = $arg->toRenderable();
+        $arg = new ReadOnlyArrayObject($arg->toRenderable());
       }
       elseif (method_exists($arg, '__toString')) {
         return (string) $arg;