diff --git a/core/src/net/sf/openrocket/document/Simulation.java b/core/src/net/sf/openrocket/document/Simulation.java
index eebc32532..5a8b8ed23 100644
--- a/core/src/net/sf/openrocket/document/Simulation.java
+++ b/core/src/net/sf/openrocket/document/Simulation.java
@@ -435,6 +435,7 @@ public class Simulation implements ChangeSource, Cloneable {
 			copy.mutex = SafetyMutex.newInstance();
 			copy.status = Status.NOT_SIMULATED;
 			copy.options = this.options.clone();
+			copy.simulationExtensions = new ArrayList<SimulationExtension>();
 			for (SimulationExtension c : this.simulationExtensions) {
 				copy.simulationExtensions.add(c.clone());
 			}