How to properly use JWT secret from environment variables in TypeScript (Node.js)?

I’m writing a Mongoose user schema in TypeScript, and inside the schema methods, I want to generate JWT access tokens and refresh tokens.

I installed the required packages using:

npm i jsonwebtoken  
npm i -D @types/jsonwebtoken

However, when I call jwt.sign(), I get this TypeScript error:

I wrote this code inside my Mongoose user schema method to generate an access token:

userSchema.methods.generateAccessToken = function (
  this: IUseSchema & mongoose.Document,
): string {
  if (!this._id) {
    throw new ApiError(400, "Payload ID is missing");
  }

  if (!env.ACCESS_TOKEN_KEY) {
    throw new ApiError(400, "ACCESS_TOKEN_KEY is missing in env");
  }

  if (!env.ACCESS_TOKEN_EXPIRY) {
    throw new ApiError(400, "ACCESS_TOKEN_EXPIRY is missing in env");
  }

  const secretKey = env.ACCESS_TOKEN_KEY as Secret;
  const expiresIn = env.ACCESS_TOKEN_EXPIRY as string;

  return jwt.sign({ id: this._id }, secretKey, { expiresIn });
};

But I get an error on the line with jwt.sign.

I don’t understand what type safety I am missing here. Can someone explain what might be wrong?

No overload matches this call.
  Overload 1 of 5, '(payload: string | object | Buffer<ArrayBufferLike>, secretOrPrivateKey: null, options?: (SignOptions & { algorithm: "none"; }) | undefined): string', gave the following error.
    Argument of type 'Secret' is not assignable to parameter of type 'null'.
      Type 'string' is not assignable to type 'null'.
  Overload 2 of 5, '(payload: string | object | Buffer<ArrayBufferLike>, secretOrPrivateKey: Buffer<ArrayBufferLike> | Secret | PrivateKeyInput | JsonWebKeyInput, options?: SignOptions | undefined): string', gave the following error.
    Type 'string' is not assignable to type 'number | StringValue | undefined'.
  Overload 3 of 5, '(payload: string | object | Buffer<ArrayBufferLike>, secretOrPrivateKey: Buffer<ArrayBufferLike> | Secret | PrivateKeyInput | JsonWebKeyInput, callback: SignCallback): void', gave the following error.
    Object literal may only specify known properties, and 'expiresIn' does not exist in type 'SignCallback'.

I’m passing the arguments like this (example):

jwt.sign(payload, process.env.JWT_SECRET, { expiresIn: process.env.ACCESS_TOKEN_EXPIRY });

I created a jwt.sign, and in that, I added process.env.ACCESS_TOKEN_KEY and process.env.ACCESS_TOKEN_EXPIRY. But the issue is that it's not working properly in JWT. Even though I added safety measures, it still doesn't work as expected. So, I tried doing it like this: expiresIn: "30d". But since these are secure credentials (like a private key), they should be stored safely.

So my issue is: Why is it not working when I use process.env? I'm getting a "no overload" error.