diff --git a/src/index.ts b/src/index.ts
index 6d123ae..cc28e55 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -2,11 +2,11 @@ import { getToken } from "./get-token";
 
 import { Options, Result } from "./types";
 
-export async function githubAppJwt({
+export async function githubAppJwt<T extends number | string = number>({
   id,
   privateKey,
   now = Math.floor(Date.now() / 1000),
-}: Options): Promise<Result> {
+}: Options<T>): Promise<Result<T>> {
   // When creating a JSON Web Token, it sets the "issued at time" (iat) to 30s
   // in the past as we have seen people running situations where the GitHub API
   // claimed the iat would be in future. It turned out the clocks on the
diff --git a/src/types.ts b/src/types.ts
index 75746e1..dea97a2 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -1,16 +1,15 @@
 export type PrivateKey = string;
-export type AppId = number;
 export type Expiration = number;
 export type Token = string;
 
-export type Options = {
-  id: AppId;
+export type Options<IdType extends number | string = number> = {
+  id: IdType;
   privateKey: PrivateKey;
   now?: number;
 };
 
-export type Result = {
-  appId: AppId;
+export type Result<IdType extends number | string = number> = {
+  appId: IdType;
   expiration: Expiration;
   token: Token;
 };
@@ -18,7 +17,7 @@ export type Result = {
 export type Payload = {
   iat: number;
   exp: number;
-  iss: number;
+  iss: number | string;
 };
 
 export type GetTokenOptions = {
diff --git a/test/node.test.ts b/test/node.test.ts
index 1d3b643..d0abdef 100644
--- a/test/node.test.ts
+++ b/test/node.test.ts
@@ -116,14 +116,12 @@ test("Include the time difference in the expiration and issued_at field", async
 });
 
 // New test for id set to Client ID
-test("id set to Client ID", async (t) => {
-  MockDate.set(0);
-
+test("id set to Client ID", async () => {
   const result = await githubAppJwt({
     id: "client_id_string",
     privateKey: PRIVATE_KEY_PKCS8,
   });
 
-  t.is(typeof result.token, "string");
-  t.is(result.appId, "client_id_string");
+  expect(typeof result.token).toEqual("string");
+  expect(result.appId).toEqual("client_id_string");
 });