diff --git a/storage/partitioned-estimate-usage-details-caches.tentative.https.sub.html b/storage/partitioned-estimate-usage-details-caches.tentative.https.sub.html
index 2eac826eb10d2e..dc2af7c213c549 100644
--- a/storage/partitioned-estimate-usage-details-caches.tentative.https.sub.html
+++ b/storage/partitioned-estimate-usage-details-caches.tentative.https.sub.html
@@ -28,19 +28,23 @@
       `/resources/partitioned-estimate-usage-details-caches-helper-frame.html`
     document.body.appendChild(iframe);
 
+
     async_test(test => {
       if (location.origin === alt_origin)
         return;
 
+
       let cache, cache_url;
-      test.step(async () => {
-        details.init = await usageDetails();
-        [cache, cache_url] = await createSomeUsage(test);
-        details.after = await usageDetails();
-        assert_greater_than(details.after, details.init);
+      window.addEventListener("message", test.step_func(async event => {
+        if (event.data === "iframe-is-ready") {
+          details.init = await usageDetails();
+          [cache, cache_url] = await createSomeUsage(test);
+          details.after = await usageDetails();
+          assert_greater_than(details.after, details.init);
 
-        iframe.contentWindow.postMessage("get-details", iframe.origin);
-      });
+          iframe.contentWindow.postMessage("get-details", iframe.origin);
+        }
+      }));
 
       window.addEventListener("message", test.step_func(event => {
         if (event.data.source === "same-site") {
@@ -48,6 +52,7 @@
 
           const cross_site_window = window
             .open(`${alt_origin}${location.pathname}`, "", "noopener=false");
+
           test.add_cleanup(() => cross_site_window.close());
         }
         if (event.data.source === "cross-site") {
diff --git a/storage/resources/partitioned-estimate-usage-details-caches-helper-frame.html b/storage/resources/partitioned-estimate-usage-details-caches-helper-frame.html
index 13786d7ceafca7..0679c1decf5583 100644
--- a/storage/resources/partitioned-estimate-usage-details-caches-helper-frame.html
+++ b/storage/resources/partitioned-estimate-usage-details-caches-helper-frame.html
@@ -25,4 +25,6 @@
       parent.opener.postMessage(details, parent.opener.origin);
     }
   });
+
+  window.parent.postMessage("iframe-is-ready", window.parent.origin);
 </script>