diff --git a/dom/xhr/XMLHttpRequestString.h b/dom/xhr/XMLHttpRequestString.h
index b02040b7ea..4a87aa4d8a 100644
--- a/dom/xhr/XMLHttpRequestString.h
+++ b/dom/xhr/XMLHttpRequestString.h
@@ -201,8 +201,9 @@ public:
   }
 
   size_t
-  SizeOfThis(MallocSizeOf aMallocSizeOf) const
+  SizeOfThis(MallocSizeOf aMallocSizeOf)
   {
+    MutexAutoLock lock(mMutex);
     return mData.SizeOfExcludingThisIfUnshared(aMallocSizeOf);
   }