Python - Add custom user models in Django admin not working

I'm working on a project using Python(3.7) and Django(2.2) in which I have implemented my models for multiple user types with custom user model as the base model. Everything working fine except the admin side, I have register these modles to admin but when I try to add an object from admin interface it's giving an error.

Here's what I have tried so far:

From models.py:

class User(AbstractBaseUser, PermissionsMixin):
    email = models.EmailField(max_length=254, unique=True)
    title = models.CharField(max_length=255, blank=False)
    user_type = models.CharField(max_length=255, choices=USER_TYPE, blank=False)
    gender = models.CharField(max_length=255, choices=CHOICES, blank=False)
    contenst = models.CharField(max_length=255, blank=True)
    is_staff = models.BooleanField(default=False)
    is_superuser = models.BooleanField(default=False)
    is_active = models.BooleanField(default=True)
    last_login = models.DateTimeField(null=True, blank=True)
    date_joined = models.DateTimeField(auto_now_add=True)

    USERNAME_FIELD = 'email'
    EMAIL_FIELD = 'email'
    REQUIRED_FIELDS = ['password']

    objects = UserManager()

    def get_absolute_url(self):
        return "/users/%i/" % (self.pk)


class PersonalBelow18(models.Model):
    user = models.OneToOneField(User, on_delete=models.CASCADE, related_name='profile')
    dob = models.DateField(blank=False)
    customer_id = models.BigIntegerField(blank=False)
    collection_use_personal_data = models.BooleanField(blank=False)
    reference_identities = models.ForeignKey(Identities, blank=False, on_delete=models.CASCADE, related_name='refs')

    def __str__(self):
        return self.user.email+'\'s account with ' + str(self.customer_id)


class PersonalAbove18(models.Model):
    user = models.OneToOneField(User, on_delete=models.CASCADE)
    dob = models.DateField(blank=False)
    customer_id = models.BigIntegerField(blank=False)
    contact_email = models.EmailField(blank=False)
    reference_identities = models.ForeignKey(Identities, blank=False, on_delete=models.CASCADE)
    contact_no = PhoneNumberField(blank=True, help_text='Phone number must be entered in the'
                                                        'format: \'+999999999\'. Up to 15 digits allowed.')

    collection_use_personal_data = models.BooleanField(blank=False)

    def __str__(self):
        return self.user.email+'\'s account with ' + str(self.customer_id)


class Parent(models.Model):
    user = models.ForeignKey(User, on_delete=models.CASCADE)
    contact_email = models.EmailField(blank=False)
    customer_id = models.BigIntegerField(blank=True)
    contact_no = PhoneNumberField(blank=True, help_text='Phone number must be entered in the'
                                                        'format: \'+999999999\'. Up to 15 digits allowed.')
    collection_use_personal_data = models.BooleanField(blank=False)


class GroupContactPerson(models.Model):
    user = models.ForeignKey(User, on_delete=models.CASCADE)
    contact_email = models.EmailField(blank=False)
    customer_id = models.BigIntegerField(blank=False)
    contact_no = PhoneNumberField(blank=True, help_text='Phone number must be entered in the'
                                                        'format: \'+999999999\'. Up to 15 digits allowed.')
    department = models.CharField(max_length=255, blank=False)
    address = models.TextField(max_length=255, blank=False)

and here's how I register these models to admin:

From admin.py:

class UserAdmin(BaseUserAdmin):
    fieldsets = (
        (None, {'fields': ('email', 'password', 'title', 'user_type',
                           'gender', 'contenst', 'last_login')}),
        ('Permissions', {'fields': (
            'is_active', 
            'is_staff', 
            'is_superuser',
            'groups', 
            'user_permissions',
        )}),
    )
    add_fieldsets = (
        (
            None,
            {
                'classes': ('wide',),
                'fields': ('email', 'password1', 'password2')
            }
        ),
    )

    list_display = ('email', 'title', 'is_staff', 'last_login')
    list_filter = ('is_staff', 'is_superuser', 'is_active', 'groups')
    search_fields = ('email',)
    ordering = ('email',)
    filter_horizontal = ('groups', 'user_permissions',)

admin.site.register(User, UserAdmin)
admin.site.register(PersonalBelow18)
admin.site.register(PersonalAbove18)
admin.site.register(Parent)
admin.site.register(GroupContactPerson)

The Parent and GroupContactPerson models are working well on admin side but the when I try to add an object for PersonalBelow18 & PersonalAbove18 models, it gives the following error as:

TypeError: str returned non-string (type int)