diff --git a/dom/base/nsGlobalWindow.cpp b/dom/base/nsGlobalWindow.cpp
index fe286d0b5a4..f06340bf534 100644
--- a/dom/base/nsGlobalWindow.cpp
+++ b/dom/base/nsGlobalWindow.cpp
@@ -2031,7 +2031,7 @@ nsGlobalWindow::WouldReuseInnerWindow(nsIDocument* aNewDocument)
 void
 nsGlobalWindow::SetInitialPrincipalToSubject()
 {
-  FORWARD_TO_OUTER_VOID(SetInitialPrincipalToSubject, ());
+  MOZ_ASSERT(IsOuterWindow());
 
   // First, grab the subject principal.
   nsCOMPtr<nsIPrincipal> newWindowPrincipal = nsContentUtils::SubjectPrincipal();
diff --git a/dom/base/nsGlobalWindow.h b/dom/base/nsGlobalWindow.h
index 8299d17c383..802ae5169e9 100644
--- a/dom/base/nsGlobalWindow.h
+++ b/dom/base/nsGlobalWindow.h
@@ -434,6 +434,7 @@ public:
   virtual NS_HIDDEN_(void) SetIsBackground(bool aIsBackground);
   virtual NS_HIDDEN_(void) SetChromeEventHandler(mozilla::dom::EventTarget* aChromeEventHandler);
 
+  // Outer windows only.
   virtual NS_HIDDEN_(void) SetInitialPrincipalToSubject();
 
   virtual NS_HIDDEN_(PopupControlState) PushPopupControlState(PopupControlState state, bool aForce) const;
diff --git a/dom/base/nsPIDOMWindow.h b/dom/base/nsPIDOMWindow.h
index 408275bbd1c..99b44e0e379 100644
--- a/dom/base/nsPIDOMWindow.h
+++ b/dom/base/nsPIDOMWindow.h
@@ -278,6 +278,7 @@ public:
 
   // Set the window up with an about:blank document with the current subject
   // principal.
+  // Outer windows only.
   virtual void SetInitialPrincipalToSubject() = 0;
 
   virtual PopupControlState PushPopupControlState(PopupControlState aState,