marshal_with for List of SQLAlchemy objects in Flask RESTful

I am trying to serialize the list of SQLAlchemy objects in JSON using marshal_with in flask_restful. For single object it worked for me.

from flask import Flask
from flask_sqlalchemy import SQLAlchemy
from flask_restful import Resource, Api, marshal_with, reqparse, fields

app = Flask(__name__)
db = SQLAlchemy(app)
api = Api(app)

app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:///data.db'
app.config['SQLALCHEMY_TRACK_MODIFICATIONS'] = False

class Person(db.Model):
  id = db.Column(db.Integer, primary_key=True)
  name = db.Column(db.String(30), nullable=False)

  pets = db.relationship('Pet', backref='owner')

  def __init__(self,name):
    self.name = name

  @classmethod
  def find_by_id(cls, id):
    return cls.query.filter_by(id=id).first()

  def save_to_db(self):
    db.session.add(self)
    db.session.commit()

  def delete_from_db(self):
    db.session.delete(self)
    db.session.commit()


class Pet(db.Model):
  id = db.Column(db.Integer, primary_key=True)
  name = db.Column(db.String(20), nullable=False)
  owner_id = db.Column(db.Integer, db.ForeignKey('person.id'))

  def __init__(self,name,owner_id):
    self.name = name
    self.owner_id = owner_id

  @classmethod
  def find_by_id(cls, id):
    return cls.query.filter_by(id=id).first()

  def save_to_db(self):
    db.session.add(self)
    db.session.commit()

  def delete_from_db(self):
    db.session.delete(self)
    db.session.commit()

pet_serializer = {
  'id':fields.Integer,
  'name':fields.String
}

person_serializer = {
  'id':fields.Integer,
  'name':fields.String,
  'pets':fields.Nested(pet_serializer)
}

person_list_serializer = {
  'items':fields.List(fields.Nested(person_serializer))
}

class PersonController(Resource):
  @marshal_with(person_serializer)
  def get(self,id):
    person = Person.find_by_id(id)
    if person:
        return person, 200
    else:
        return {'msg':'Resource not found'}, 404

class PersonListController(Resource):
  @marshal_with(person_list_serializer)
  def get(self):
    return Person.query.all()

api.add_resource(PersonController, '/person/<int:id>')
api.add_resource(PersonListController, '/persons')

if __name__ == '__main__':
  app.run(debug=True, port=5000)

The API response for the PersonController get request is as:

{
"id": 1,
"name": "Leo",
"pets": [
    {
        "id": 1,
        "name": "Spoc"
    }
  ]
}

but for the PersonListController get request I am getting empty response as:

[
  {
    "items": null
  },
  {
    "items": null
  }
]

Please can anyone help me about the issue here.