diff --git a/src/concurrent/qtconcurrentreducekernel.h b/src/concurrent/qtconcurrentreducekernel.h
index a58739fc410..a2c693d9e2e 100644
--- a/src/concurrent/qtconcurrentreducekernel.h
+++ b/src/concurrent/qtconcurrentreducekernel.h
@@ -187,11 +187,13 @@ public:
 
     inline bool shouldThrottle()
     {
+        std::lock_guard<QMutex> locker(mutex);
         return (resultsMapSize > (ReduceQueueThrottleLimit * threadCount));
     }
 
     inline bool shouldStartThread()
     {
+        std::lock_guard<QMutex> locker(mutex);
         return (resultsMapSize <= (ReduceQueueStartLimit * threadCount));
     }
 };