diff --git a/src/benchmark.cc b/src/benchmark.cc
index de8d44d4..1ad2e32a 100644
--- a/src/benchmark.cc
+++ b/src/benchmark.cc
@@ -577,6 +577,7 @@ class State::FastClock {
   // function starts running - see UseRealTime).
   void InitType(Type type) {
     type_ = type;
+    mutex_lock l(&bg_mutex_);
     std::atomic_store(&approx_time_, NowMicros());
   }