diff --git a/src/Subscription.ts b/src/Subscription.ts
index 3fcf43c5a7..aec99264ec 100644
--- a/src/Subscription.ts
+++ b/src/Subscription.ts
@@ -13,7 +13,7 @@ export type TeardownLogic = AnonymousSubscription | Function | void;
 
 export interface ISubscription extends AnonymousSubscription {
   unsubscribe(): void;
-  closed: boolean;
+  readonly closed: boolean;
 }
 
 /**