diff --git a/core/lib/Drupal/Core/Ajax/CommandWithAttachedAssetsTrait.php b/core/lib/Drupal/Core/Ajax/CommandWithAttachedAssetsTrait.php
index 319e3a4..c8e3b83 100644
--- a/core/lib/Drupal/Core/Ajax/CommandWithAttachedAssetsTrait.php
+++ b/core/lib/Drupal/Core/Ajax/CommandWithAttachedAssetsTrait.php
@@ -33,6 +33,7 @@
    *   HTML rendered content.
    */
   protected function getRenderedContent() {
+    $this->attachedAssets = new AttachedAssets();
     if (is_array($this->content)) {
       $html = \Drupal::service('renderer')->render($this->content);
       $this->attachedAssets = AttachedAssets::createFromRenderArray($this->content);