diff --git a/jsprit-core/src/main/java/com/graphhopper/jsprit/core/problem/job/Service.java b/jsprit-core/src/main/java/com/graphhopper/jsprit/core/problem/job/Service.java
index 436914b1..c16a74db 100644
--- a/jsprit-core/src/main/java/com/graphhopper/jsprit/core/problem/job/Service.java
+++ b/jsprit-core/src/main/java/com/graphhopper/jsprit/core/problem/job/Service.java
@@ -28,6 +28,7 @@ import com.graphhopper.jsprit.core.util.Coordinate;
 
 import java.util.ArrayList;
 import java.util.Collection;
+import java.util.Collections;
 import java.util.List;
 
 /**
@@ -302,6 +303,7 @@ public class Service extends AbstractJob {
         priority = builder.priority;
 	    maxTimeInVehicle = builder.maxTimeInVehicle;
         activities.add(builder.activity);
+        activities = Collections.unmodifiableList(activities);
 	}
 
     public Collection<TimeWindow> getTimeWindows(){
diff --git a/jsprit-core/src/main/java/com/graphhopper/jsprit/core/problem/job/Shipment.java b/jsprit-core/src/main/java/com/graphhopper/jsprit/core/problem/job/Shipment.java
index 4db6679e..be95482b 100644
--- a/jsprit-core/src/main/java/com/graphhopper/jsprit/core/problem/job/Shipment.java
+++ b/jsprit-core/src/main/java/com/graphhopper/jsprit/core/problem/job/Shipment.java
@@ -26,6 +26,7 @@ import com.graphhopper.jsprit.core.problem.solution.route.activity.TimeWindowsIm
 
 import java.util.ArrayList;
 import java.util.Collection;
+import java.util.Collections;
 import java.util.List;
 
 
@@ -392,6 +393,7 @@ public class Shipment extends AbstractJob {
         this.maxTimeInVehicle = builder.maxTimeInVehicle;
         activities.add(builder.pickup);
         activities.add(builder.delivery);
+        activities = Collections.unmodifiableList(activities);
     }
 
     @Override