diff --git a/src/client/sound/Sounds.ts b/src/client/sound/Sounds.ts
index 4646ea04d..34ab8fc4a 100644
--- a/src/client/sound/Sounds.ts
+++ b/src/client/sound/Sounds.ts
@@ -51,7 +51,7 @@ export class PlaySoundEffectEvent implements GameEvent {
 }
 
 export class SoundUpdateEvent implements GameEvent {
-  constructor(public gu: GameUpdateViewData) {}
+  constructor(public readonly gu: GameUpdateViewData) {}
 }
 
 export class SetSoundEffectsVolumeEvent implements GameEvent {