--- a/dom/ipc/tests/test_child_docshell.html
+++ b/dom/ipc/tests/test_child_docshell.html
@@ -71,17 +71,25 @@ SpecialPowers.pushPrefEnv({'set':[
                                 .QueryInterface(Ci.nsIDocShell);
       sendAsyncMessage("DOMWindowCreatedReceived", {
         stableChromeEventHandler: chromeEventHandler === docShell.chromeEventHandler,
         iframeHasNewDocShell: new_docShell !== docShell,
         iframeHasSameChromeEventHandler: new_docShell.chromeEventHandler === chromeEventHandler
       });
     });
 
-    let i = content.document.createElement("iframe");
-    i.setAttribute("src", "data:text/html,foo");
-    content.document.documentElement.appendChild(i);
+    function go() {
+      let i = content.document.createElement("iframe");
+      i.setAttribute("src", "data:text/html,foo");
+      content.document.documentElement.appendChild(i);
+    }
+
+    if (content.document.readyState == "complete") {
+      go();
+    } else {
+      addEventListener("load", go, { once: true, capture: true });
+    }
   }, false);
 });
 
 </script>
 </body>
 </html>