diff --git a/achartengine/src/org/achartengine/model/XYSeries.java b/achartengine/src/org/achartengine/model/XYSeries.java
index e39a471..8072693 100644
--- a/achartengine/src/org/achartengine/model/XYSeries.java
+++ b/achartengine/src/org/achartengine/model/XYSeries.java
@@ -334,7 +334,7 @@ public synchronized SortedMap<Double, Double> getRange(double start, double stop
     }
   }
 
-  public int getIndexForKey(double key) {
+  public synchronized int getIndexForKey(double key) {
     return mXY.getIndexForKey(key);
   }