I'm trying to add an attribute to the object I've created in Django shell to create some queries but it says the object is not itrable

I'm trying to make a project with django I've migrated the model successfully and when I want to add account number to my checkingaccount objects it says the object is not iterable everything else works properly(as I tested)

this is the models.py:

 from django.db import models

# Create your models here.

class Bank(models.Model):
    name = models.CharField(max_length=300)
    
    def __str__(self) -> str:
        return self.name

class BankAccount(models.Model):
    bank =            models.ForeignKey(Bank, on_delete=models.CASCADE)
    balance =         models.DecimalField(max_digits=18, decimal_places=2)
    account_number =  models.IntegerField()
    types = (
        ('c', 'checking account'),
        ('s', 'savings account'),
    )
    type_selection =  models.CharField('account type', max_length=1, choices=types )
    
    def __str__(self) -> str:
        return str(self.account_number)

class CheckingAccount(models.Model):
    balance =           models.DecimalField(max_digits=18, decimal_places=2)
    date_created =      models.DateField()
    last_transaction =  models.DateField()
    account_number =    models.ManyToManyField(BankAccount)
    
    def __str__(self) -> str:
        return self.account_number
    
    def deposit(cash, self):
        self.balance += cash
    
    def check_balance(self):
        return self.balance
    
    def withdrawal(self, cash):
        self.balance -= cash

class SavingsAccount(models.Model):
    balance =         models.DecimalField(max_digits=18, decimal_places=2)
    date_created =    models.DateField()
    account_number =  models.ManyToManyField(BankAccount)
    
    def __str__(self) -> str:
        return self.account_number
    
    def deposit(cash, self):
        self.balance += cash
    
    def check_balance(self):
        return self.balance

this is the shell input and output:

>>> acC.account_number.set(ac)
Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "C:\Users\Armin\AppData\Local\Programs\Python\Python39\lib\site-packages\django\db\models\fields\related_descriptors.py", line 992, in set
    objs = tuple(objs)
TypeError: 'BankAccount' object is not iterable

any helps appreciated =)