Index: binaries/data/mods/public/simulation/helpers/Commands.js
===================================================================
--- binaries/data/mods/public/simulation/helpers/Commands.js
+++ binaries/data/mods/public/simulation/helpers/Commands.js
@@ -8,6 +8,10 @@
 	if (!cmpPlayer)
 		return;
 
+	// Commands are frozen, but we may need to serialize them and that requires extensible
+	// objects, so clone them.
+	cmd = clone(cmd);
+
 	let data = {
 		"cmpPlayer": cmpPlayer,
 		"controlAllUnits": cmpPlayer.CanControlAllUnits()