diff --git a/core/src/main/java/com/yahoo/ycsb/measurements/OneMeasurementTimeSeries.java b/core/src/main/java/com/yahoo/ycsb/measurements/OneMeasurementTimeSeries.java
index 34805bc428b5b872caedc6cc2780ede0395727e8..5e8dea01f8d1324c93e151d11476480d15ccefbd 100644
--- a/core/src/main/java/com/yahoo/ycsb/measurements/OneMeasurementTimeSeries.java
+++ b/core/src/main/java/com/yahoo/ycsb/measurements/OneMeasurementTimeSeries.java
@@ -76,7 +76,7 @@ public class OneMeasurementTimeSeries extends OneMeasurement
     _measurements=new Vector<SeriesUnit>();
   }
 
-  void checkEndOfUnit(boolean forceend)
+  synchronized void checkEndOfUnit(boolean forceend)
   {
     long now=System.currentTimeMillis();