diff --git a/OpenRA.Mods.Common/Traits/Render/WithRepairOverlay.cs b/OpenRA.Mods.Common/Traits/Render/WithRepairOverlay.cs
index bb3d8b9b9b..fccd3e285f 100644
--- a/OpenRA.Mods.Common/Traits/Render/WithRepairOverlay.cs
+++ b/OpenRA.Mods.Common/Traits/Render/WithRepairOverlay.cs
@@ -37,7 +37,7 @@ namespace OpenRA.Mods.Common.Traits
 
 	public class WithRepairOverlay : INotifyDamageStateChanged, INotifyBuildComplete, INotifySold, INotifyRepair
 	{
-		Animation overlay;
+		readonly Animation overlay;
 		bool buildComplete;
 
 		public WithRepairOverlay(Actor self, WithRepairOverlayInfo info)