diff --git a/OpenRA.Mods.Common/Scripting/Global/MapGlobal.cs b/OpenRA.Mods.Common/Scripting/Global/MapGlobal.cs
index 7f6c4fa2ca..0a90f3c191 100644
--- a/OpenRA.Mods.Common/Scripting/Global/MapGlobal.cs
+++ b/OpenRA.Mods.Common/Scripting/Global/MapGlobal.cs
@@ -19,7 +19,7 @@ namespace OpenRA.Mods.Common.Scripting
 	[ScriptGlobal("Map")]
 	public class MapGlobal : ScriptGlobal
 	{
-		SpawnMapActors sma;
+		readonly SpawnMapActors sma;
 		public MapGlobal(ScriptContext context)
 			: base(context)
 		{