diff --git a/source/network/NetTurnManager.cpp b/source/network/NetTurnManager.cpp
index 9cbcf4b179..60eade6f33 100644
--- a/source/network/NetTurnManager.cpp
+++ b/source/network/NetTurnManager.cpp
@@ -1,4 +1,4 @@
-/* Copyright (C) 2015 Wildfire Games.
+/* Copyright (C) 2016 Wildfire Games.
  * This file is part of 0 A.D.
  *
  * 0 A.D. is free software: you can redistribute it and/or modify
@@ -298,6 +298,7 @@ void CNetTurnManager::AddCommand(int client, int player, JS::HandleValue data, u
 		return;
 	}
 
+	m_Simulation2.GetScriptInterface().FreezeObject(data, true);
 	m_QueuedCommands[turn - (m_CurrentTurn+1)][client].emplace_back(player, m_Simulation2.GetScriptInterface().GetContext(), data);
 }
 
diff --git a/source/simulation2/Simulation2.cpp b/source/simulation2/Simulation2.cpp
index 9ce2138135..18618f73b3 100644
--- a/source/simulation2/Simulation2.cpp
+++ b/source/simulation2/Simulation2.cpp
@@ -164,6 +164,7 @@ public:
 			JSContext* cxNew = newScript.GetContext();
 			JSAutoRequest rqNew(cxNew);
 			JS::RootedValue tmpCommand(cxNew, newScript.CloneValueFromOtherContext(oldScript, command.data));
+			newScript.FreezeObject(tmpCommand, true);
 			SimulationCommand cmd(command.player, cxNew, tmpCommand);
 			newCommands.emplace_back(std::move(cmd));
 		}