diff --git a/testing/web-platform/tests/tools/wptrunner/wptrunner/executors/test-wait.js b/testing/web-platform/tests/tools/wptrunner/wptrunner/executors/test-wait.js
index 8a7edb79d622..ad08ad7d76fb 100644
--- a/testing/web-platform/tests/tools/wptrunner/wptrunner/executors/test-wait.js
+++ b/testing/web-platform/tests/tools/wptrunner/wptrunner/executors/test-wait.js
@@ -32,7 +32,8 @@ function wait_paints() {
 }
 
 function screenshot_if_ready() {
-  if (root.classList.contains("%(classname)s") &&
+  if (root &&
+      root.classList.contains("%(classname)s") &&
       observer === null) {
     observer = new MutationObserver(wait_paints);
     observer.observe(root, {attributes: true});