1 files changed, 7 insertions, 3 deletions
diff --git a/testing/web-platform/tests/credential-management/support/fencedframe-mark-signedin.html b/testing/web-platform/tests/credential-management/support/fencedframe-mark-signedin.html
index 532db7047a..681fcd6787 100644
--- a/testing/web-platform/tests/credential-management/support/fencedframe-mark-signedin.html
+++ b/testing/web-platform/tests/credential-management/support/fencedframe-mark-signedin.html
@@ -3,13 +3,17 @@
 
 <fencedframe></fencedframe>
 <script>
-const url = new URL("mark_signedin", location.href);
-document.querySelector("fencedframe").config = new FencedFrameConfig(url);
-
 // If this page was opened as a popup, notify the opener when we are done loading.
 if (window.opener) {
   window.onload = function() {
     window.opener.postMessage("done_loading", "*");
   };
 }
+
+// This code is intentionally after the onload listener registration
+// because it can throw if FencedFrameConfig is not defined. In that
+// case, we still want to notify the opener to avoid a test timeout.
+const url = new URL("mark_signedin", location.href);
+document.querySelector("fencedframe").config = new FencedFrameConfig(url);
+
 </script>