diff --git a/dom/base/nsDocument.cpp b/dom/base/nsDocument.cpp
index b0c148b052..513f9b7a93 100644
--- a/dom/base/nsDocument.cpp
+++ b/dom/base/nsDocument.cpp
@@ -4668,8 +4668,16 @@ nsDocument::SetScriptGlobalObject(nsIScriptGlobalObject *aScriptGlobalObject)
     }
   }
 
+  // BlockOnload() might be called before mScriptGlobalObject is set.
+  // We may need to add the blocker once mScriptGlobalObject is set.
+  bool needOnloadBlocker = !mScriptGlobalObject && aScriptGlobalObject;
+
   mScriptGlobalObject = aScriptGlobalObject;
 
+  if (needOnloadBlocker) {
+    EnsureOnloadBlocker();
+  }
+
   if (aScriptGlobalObject) {
     mHasHadScriptHandlingObject = true;
     mHasHadDefaultView = true;