CastError when querying MongoDB with Mongoose

I am getting a CastError (Cast to ObjectId failed for value) when querying a MongoDB database with a Mongoose model. Is there a solution to checking whether a user is found, and if not, continuing with the code whilst excluding the user object? The id is passed in the correct format.

export async function getServerSideProps(context) {
  const session = await getSession(context);
  await dbConnect();

  const id = context.params.id;
  const user = await User.findById(id).lean();
  if (user) user._id = user._id.toString(); //if user is found

  return { props: { user, session } };
}