diff --git a/layout/base/nsDocumentViewer.cpp b/layout/base/nsDocumentViewer.cpp
index 96cfb4808ca..e9ebc835fe1 100644
--- a/layout/base/nsDocumentViewer.cpp
+++ b/layout/base/nsDocumentViewer.cpp
@@ -2315,7 +2315,11 @@ DocumentViewerImpl::FindContainerView()
           parentDocShell->GetPresShell(getter_AddRefs(parentPresShell));
         }
       }
-      if (content && parentPresShell) {
+      if (!content) {
+        NS_WARNING("Subdocument container has no content");
+      } else if (!parentPresShell) {
+        NS_WARNING("Subdocument container has no presshell");
+      } else {
         nsIFrame* f = parentPresShell->GetRealPrimaryFrameFor(content);
         if (f) {
           nsIFrame* subdocFrame = f->GetContentInsertionFrame();
@@ -2329,7 +2333,11 @@ DocumentViewerImpl::FindContainerView()
             nsIView* innerView = subdocFrameView->GetFirstChild();
             NS_ASSERTION(innerView, "Subdoc frames must have an inner view too");
             containerView = innerView;
+          } else {
+            NS_WARNING("Subdocument container has non-subdocument frame");
           }
+        } else {
+          NS_WARNING("Subdocument container has no frame");
         }
       }
     }
diff --git a/widget/src/windows/nsWindow.cpp b/widget/src/windows/nsWindow.cpp
index fc2b283eb41..481b8fb8f27 100644
--- a/widget/src/windows/nsWindow.cpp
+++ b/widget/src/windows/nsWindow.cpp
@@ -642,6 +642,9 @@ nsWindow::StandardWindowCreate(nsIWidget *aParent,
       extendedStyle = WS_EX_TOOLWINDOW;
     else
       parent = NULL;
+  } else if (mWindowType == eWindowType_invisible) {
+    // Make sure CreateWindowEx succeeds at creating a toplevel window
+    style &= ~WS_CHILDWINDOW;
   } else if (nsnull != aInitData) {
     // See if the caller wants to explictly set clip children and clip siblings
     if (aInitData->clipChildren) {