diff --git a/src/logic/navigator.ts b/src/logic/navigator.ts
index fb6e1cf..01b4c89 100644
--- a/src/logic/navigator.ts
+++ b/src/logic/navigator.ts
@@ -8,7 +8,7 @@ export interface Index
 export default class Navigator
 {
     private activeIndex: Index;
-    private updateCallback: (() => void) | undefined;
+    private updateCallback?: () => void;
 
     /**
      * Update index