src/Renderer.php | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/src/Renderer.php b/src/Renderer.php
index 48bc491..3e40044 100644
--- a/src/Renderer.php
+++ b/src/Renderer.php
@@ -31,6 +31,13 @@ class Renderer extends CoreRenderer {
       return '';
     }
 
+    // When the result is not HTML, renderviz can't visualize it.
+    // @see \Drupal\big_pipe\Render\Placeholder\BigPipeStrategy::doProcessPlaceholders()
+    $output = ltrim((string) $elements['#markup']);
+    if ($output[0] !== '<') {
+      return $result;
+    }
+
     // Apply the same default cacheability logic that Renderer::doRender()
     // applies.
     $pre_bubbling_elements = $original_elements;