diff --git a/dom/base/EventSource.cpp b/dom/base/EventSource.cpp
index 06cabc3098..32329098e2 100644
--- a/dom/base/EventSource.cpp
+++ b/dom/base/EventSource.cpp
@@ -190,6 +190,7 @@ public:
 
   bool IsClosed()
   {
+    MutexAutoLock lock(mMutex); 
     return ReadyState() == CLOSED;
   }