--- a/dom/ipc/TabChild.cpp
+++ b/dom/ipc/TabChild.cpp
@@ -1480,16 +1480,22 @@ TabChild::ZoomToRect(const uint32_t& aPr
   if (mApzcTreeManager) {
     mApzcTreeManager->ZoomToRect(guid, aRect, aFlags);
   }
 }
 
 mozilla::ipc::IPCResult
 TabChild::RecvActivate()
 {
+  // Ensure that the PresShell exists, otherwise focusing
+  // is definitely not going to work. GetPresShell should
+  // create a PresShell if one doesn't exist yet.
+  nsCOMPtr<nsIPresShell> presShell = GetPresShell();
+  MOZ_ASSERT(presShell);
+
   nsCOMPtr<nsIWebBrowserFocus> browser = do_QueryInterface(WebNavigation());
   browser->Activate();
   return IPC_OK();
 }
 
 mozilla::ipc::IPCResult
 TabChild::RecvDeactivate()
 {