TypeError: unsupported operand type(s) for +: 'int' and 'bytes'

I have a table in sqlite called User and I want to check for token so that I can reset password. I get the error found in the title. here is the code:

from datetime import datetime
from itsdangerous import Serializer as Serializer
from flaskblog import db, login_manager, app
from flask_login import UserMixin



class User(db.Model, UserMixin):
    id = db.Column(db.Integer, primary_key=True)
    username = db.Column(db.String(20), unique=True, nullable=False)
    email = db.Column(db.String(120), unique=True, nullable=False)
    image_file = db.Column(db.String(20), nullable=False, default='default.jpg')
    password = db.Column(db.String(60), nullable=False)
    posts = db.relationship('Post', backref='author', lazy=True)

    def get_reset_token(self, expires_sec=1800):
        s = Serializer(app.config['SECRET_KEY'], expires_sec)
        return s.dumps({'user_id': self.id}).decode('utf-8')

    @staticmethod
    def verify_reset_token(token):
        s = Serializer(app.config['SECRET_KEY'])
        try:
            user_id = s.loads(token)['user_id']
        except:
            return None
        return User.query.get(user_id)

    def __repr__(self):
        return f"User('{self.username}', '{self.email}', '{self.image_file}')"

the error is probably in this line but I read through some documentations and didn't find a way, please help.

   return s.dumps({'user_id': self.id}).decode('utf-8')