TypeError at /admin/app/doctor/add/ __str__ returned non-string (type NoneType)

В моем проекте django мой models.py:

class myCustomUser(AbstractUser):
    username = models.CharField(max_length=20, unique="True", blank=False)
    password = models.CharField(max_length=200)

    isPatient = models.BooleanField(default=False)
    isDoctor = models.BooleanField(default=False)

    firstName = models.CharField(max_length=200, blank=True, null=True)
    lastName = models.CharField(max_length=200, blank=True, null=True)
    email = models.EmailField(unique=True)
    dateOfBirth = models.CharField(max_length=200, blank=True, null=True)
    address = models.CharField(max_length=200, blank=True, null=True)
    contactNo = models.IntegerField(null=True, unique=True)

    def __str__(self):
        return self.firstName


class Doctor(models.Model):
    user = models.OneToOneField(myCustomUser, null=True, blank=True, on_delete=models.CASCADE, related_name='doctor_releted_user')
    degree = models.CharField(max_length=200, blank=True, null=True)
    speciality = models.CharField(max_length=200, blank=True, null=True)
    isApproved = models.BooleanField(default=False)

    def __str__(self):
        return self.user.firstName


class Patient(models.Model):
    user = models.OneToOneField(myCustomUser, null=True, blank=True, on_delete=models.CASCADE, related_name='patient_releted_user')
    gender = models.CharField(max_length=200, blank=True, null=True)


class Appointment(models.Model):
    pname = models.ForeignKey(Patient, on_delete=models.CASCADE, related_name='patient_releted_appointment', null=True, blank=True)
    doctor = models.ForeignKey(Doctor, null=True, blank=True, on_delete=models.CASCADE, related_name='doctor_releted_appornment')
    date = models.DateTimeField(null=True, blank=True)
    isConfiremed = models.BooleanField(default=False)

После первого migration когда я пытаюсь добавить любой doctor объект из канала администратора, он выдает ошибку типа:

TypeError at /admin/app/doctor/add/

__str__ returned non-string (type NoneType)

Пожалуйста, подскажите, как я могу это исправить?

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