diff --git a/xpfe/appshell/src/nsWebShellWindow.cpp b/xpfe/appshell/src/nsWebShellWindow.cpp
index 0a7c178d5546..8143d0182b79 100644
--- a/xpfe/appshell/src/nsWebShellWindow.cpp
+++ b/xpfe/appshell/src/nsWebShellWindow.cpp
@@ -252,7 +252,9 @@ nsWebShellWindow::WindowResized(nsIWidget* aWidget, PRInt32 aWidth, PRInt32 aHei
   }
 
   nsCOMPtr<nsIBaseWindow> shellAsWin(do_QueryInterface(mDocShell));
-  shellAsWin->SetPositionAndSize(0, 0, aWidth, aHeight, false);
+  if (shellAsWin) {
+    shellAsWin->SetPositionAndSize(0, 0, aWidth, aHeight, false);
+  }
   // Persist size, but not immediately, in case this OS is firing
   // repeated size events as the user drags the sizing handle
   if (!IsLocked())