.../src/Render/Placeholder/BigPipeStrategy.php     | 23 ++++++++++++++++------
 1 file changed, 17 insertions(+), 6 deletions(-)

diff --git a/core/modules/big_pipe/src/Render/Placeholder/BigPipeStrategy.php b/core/modules/big_pipe/src/Render/Placeholder/BigPipeStrategy.php
index 63229bc..be75fcb 100644
--- a/core/modules/big_pipe/src/Render/Placeholder/BigPipeStrategy.php
+++ b/core/modules/big_pipe/src/Render/Placeholder/BigPipeStrategy.php
@@ -107,7 +107,7 @@ public function processPlaceholders(array $placeholders) {
         continue;
       }
       else {
-        $return[$placeholder] = static::createBigPipeJsPlaceholder($placeholder_elements);
+        $return[$placeholder] = static::createBigPipeJsPlaceholder($placeholder, $placeholder_elements);
       }
     }
 
@@ -117,19 +117,30 @@ public function processPlaceholders(array $placeholders) {
   /**
    * Creates a BigPipe JS placeholder.
    *
+   * @param string $original_placeholder
+   *   The original placeholder.
    * @param array $placeholder_render_array
    *   The render array for a placeholder.
    *
    * @return array
    *   The resulting BigPipe JS placeholder render array.
    */
-  protected static function createBigPipeJsPlaceholder(array $placeholder_render_array) {
+  protected static function createBigPipeJsPlaceholder($original_placeholder, array $placeholder_render_array) {
     // Generate a BigPipe selector (to be used by BigPipe's JavaScript).
     // @see \Drupal\Core\Render\PlaceholderGenerator::createPlaceholder()
-    $callback = $placeholder_render_array['#lazy_builder'][0];
-    $arguments = $placeholder_render_array['#lazy_builder'][1];
-    $token = hash('crc32b', serialize($placeholder_render_array));
-    $big_pipe_js_selector = UrlHelper::buildQuery(['callback' => $callback, 'args' => $arguments, 'token' => $token]);
+    if (isset($placeholder_render_array['#lazy_builder'])) {
+      $callback = $placeholder_render_array['#lazy_builder'][0];
+      $arguments = $placeholder_render_array['#lazy_builder'][1];
+      $token = hash('crc32b', serialize($placeholder_render_array));
+      $big_pipe_js_selector = UrlHelper::buildQuery(['callback' => $callback, 'args' => $arguments, 'token' => $token]);
+    }
+    // When the placeholder's render array is not using a #lazy_builder,
+    // anything could be in there: only #lazy_builder has a strict contract that
+    // allows us to create a more sane selector. Therefore, simply the original
+    // placeholder into a usable selector, at the cost of it being obtuse.
+    else {
+      $big_pipe_js_selector = Html::getId($original_placeholder);
+    }
 
     return [
       '#markup' => '<div data-big-pipe-selector="' . Html::escape($big_pipe_js_selector) . '"></div>',