diff --git a/internal/ceres/wall_time.h b/internal/ceres/wall_time.h
index 1dbd6e9..45f65ca 100644
--- a/internal/ceres/wall_time.h
+++ b/internal/ceres/wall_time.h
@@ -72,7 +72,7 @@
 //     Total:  time3  time1 + time2 + time3;
 class EventLogger {
  public:
-  EventLogger(const string& logger_name);
+  explicit EventLogger(const string& logger_name);
   ~EventLogger();
   void AddEvent(const string& event_name);