diff --git a/src/browser/services/DecorationService.ts b/src/browser/services/DecorationService.ts
index 992cd339..cc059d21 100644
--- a/src/browser/services/DecorationService.ts
+++ b/src/browser/services/DecorationService.ts
@@ -61,9 +61,9 @@ export class DecorationService extends Disposable implements IDecorationService
 export const IDecorationService = createDecorator<IDecorationService>('DecorationService');
 class Decoration extends Disposable implements IDecoration {
   private static _nextId = 1;
-  private _marker: IMarker;
+  private readonly _marker: IMarker;
   private _element: HTMLElement | undefined;
-  private _id: number = Decoration._nextId++;
+  private readonly _id: number = Decoration._nextId++;
   public isDisposed: boolean = false;
 
   public get id(): number { return this._id; }