diff --git a/packages/core/src/core/UnCancelToken.test.ts b/packages/core/src/core/UnCancelToken.test.ts
index 797b21c..cc13444 100644
--- a/packages/core/src/core/UnCancelToken.test.ts
+++ b/packages/core/src/core/UnCancelToken.test.ts
@@ -31,6 +31,7 @@ describe("core:UnCancelToken", () => {
       cancel("Operation has been canceled.");
       expect(token.reason).toEqual(expect.any(UnCanceledError));
       expect(token.reason?.message).toBe("Operation has been canceled.");
+      expect(token.reason?.isUnCanceledError).toBe(true);
     });
 
     it("returns undefined if cancellation has not been requested", () => {
diff --git a/packages/core/src/core/UnCancelToken.ts b/packages/core/src/core/UnCancelToken.ts
index 616253e..5de374a 100644
--- a/packages/core/src/core/UnCancelToken.ts
+++ b/packages/core/src/core/UnCancelToken.ts
@@ -3,6 +3,7 @@ import { UnCanceledError } from "./UnCanceledError";
 
 export interface UnCancel {
   message?: string;
+  isUnCanceledError: true;
 }
 
 export interface UnCancelStatic {
diff --git a/packages/core/src/core/UnCanceledError.ts b/packages/core/src/core/UnCanceledError.ts
index 6b127a1..3c5eccb 100644
--- a/packages/core/src/core/UnCanceledError.ts
+++ b/packages/core/src/core/UnCanceledError.ts
@@ -2,7 +2,7 @@ import type { UnConfig, UnData, UnTask } from "../types";
 import { UnError } from "./UnError";
 
 class UnCanceledError<T = UnData, D = UnData> extends UnError<T, D> {
-  isUnCanceledError = true;
+  isUnCanceledError: true = true;
 
   constructor(message?: string, config?: UnConfig<T, D>, task?: UnTask) {
     super(message ?? "canceled");