diff --git a/embedding/browser/webBrowser/nsDocShellTreeOwner.cpp b/embedding/browser/webBrowser/nsDocShellTreeOwner.cpp
index ad0fcbdff5f..a2bde39f997 100644
--- a/embedding/browser/webBrowser/nsDocShellTreeOwner.cpp
+++ b/embedding/browser/webBrowser/nsDocShellTreeOwner.cpp
@@ -688,8 +688,7 @@ NS_IMETHODIMP nsDocShellTreeOwner::SetWebBrowserChrome(nsIWebBrowserChrome* aWeb
       nsCOMPtr<nsIWebBrowserSiteWindow> ownerWin(do_QueryInterface(aWebBrowserChrome));
       nsCOMPtr<nsIInterfaceRequestor> requestor(do_QueryInterface(aWebBrowserChrome));
 
-      NS_ENSURE_TRUE(ownerWin, NS_ERROR_INVALID_ARG);
-
+      // it's ok for ownerWin or requestor to be null.
       mWebBrowserChrome = aWebBrowserChrome;
       mOwnerWin = ownerWin;
       mOwnerRequestor = requestor;