diff --git a/Parallel/Core/vtkThreadedTaskQueue.txx b/Parallel/Core/vtkThreadedTaskQueue.txx
index ac4b2584d7536137cabced7b0aaff3ed54bdbd50..91c2368257568288c4123797b0cfdebf4de631eb 100644
--- a/Parallel/Core/vtkThreadedTaskQueue.txx
+++ b/Parallel/Core/vtkThreadedTaskQueue.txx
@@ -40,7 +40,10 @@ public:
 
   void MarkDone()
   {
-    this->Done = true;
+    {
+      std::lock_guard<std::mutex> lock(this->TasksMutex);
+      this->Done = true;
+    }
     this->TasksCV.notify_all();
   }