--- a/dom/base/nsGlobalWindow.cpp
+++ b/dom/base/nsGlobalWindow.cpp
@@ -1385,17 +1385,17 @@ nsGlobalWindow::UnmarkGrayTimers()
 {
   for (nsTimeout* timeout = mTimeouts.getFirst();
        timeout;
        timeout = timeout->getNext()) {
     if (timeout->mScriptHandler) {
       Function* f = timeout->mScriptHandler->GetCallback();
       if (f) {
         // Callable() already does xpc_UnmarkGrayObject.
-        JSObject* o = f->Callable();
+        DebugOnly<JSObject*> o = f->Callable();
         MOZ_ASSERT(!xpc_IsGrayGCThing(o), "Should have been unmarked");
       }
     }
   }
 }
 
 //*****************************************************************************
 // nsGlobalWindow::nsIScriptGlobalObject