--- a/dom/base/nsGlobalWindow.cpp
+++ b/dom/base/nsGlobalWindow.cpp
@@ -1767,17 +1767,20 @@ nsGlobalWindow::SetNewDocument(nsIDocume
 
   /* No mDocShell means we're already been partially closed down.  When that
      happens, setting status isn't a big requirement, so don't. (Doesn't happen
      under normal circumstances, but bug 49615 describes a case.) */
 
   nsContentUtils::AddScriptRunner(
     NS_NewRunnableMethod(this, &nsGlobalWindow::ClearStatus));
 
-  bool reUseInnerWindow = aForceReuseInnerWindow || wouldReuseInnerWindow;
+  // Sometimes, WouldReuseInnerWindow() returns true even if there's no inner
+  // window (see bug 776497). Be safe.
+  bool reUseInnerWindow = (aForceReuseInnerWindow || wouldReuseInnerWindow) &&
+                          GetCurrentInnerWindowInternal();
 
   nsresult rv = NS_OK;
 
   // Set mDocument even if this is an outer window to avoid
   // having to *always* reach into the inner window to find the
   // document.
   mDocument = do_QueryInterface(aDocument);
   mDoc = aDocument;