Calculating a field value based on PK in Django

Question

Goal: Set customer_number to a calculated value of b_country + id + 10000.

models.py

class Customer(models.Model):

    id = models.BigAutoField(primary_key=True)
    customer_number = models.CharField("Customer #", max_length=10, default=0)
    b_country = models.TextField("Country", max_length=2, choices=COUNTRY_OPTIONS)

    @property
    def get_customer_number(self):
        a = self.b_country
        b = (self.id + 10000)
        return a + str(b)

    def save(self, *args, **kwarg):
        self.customer_number = self.get_customer_number
        super(Customer, self).save(*args, **kwarg)

Issue: When the form is submitted to create a new customer (new record) then I get the following error:

a 'CA'
self Error in formatting: TypeError: unsupported operand type(s) for +: 'NoneType' and 'int'>

When I already have a record (updating and existing Customer's info) then the code works fine. It is my understanding that it isn't able to get the id as it is still validating the data before being sent to the model, hence no Customer.id to pull for this instance.

Question: Is there a cleaner way of setting a customer_number that is based on the PK?