RelatedObjectDoesNotExist with TabularInline and proxy models

my app's models include the Service model along with three proxy models:

class Service(models.Model):
    SERVICE_TYPE_CHOICES = [
        ('AR', 'Area'),
        ('OC', 'Occupancy'),
        ('CO', 'Consumption'),
    ]
    service_type = models.CharField(max_length=2, choices=SERVICE_TYPE_CHOICES)
    building = models.ForeignKey(Building, on_delete=models.CASCADE)

    def __init__(self, *args, **kwargs): 
        super(HouseService, self).__init__(*args, **kwargs) 
        subclasses = {
            'AR' : AreaService,
            'OC' : OccupancyService,
            'CO' : ConsumptionService,
        }
        self.__class__ = subclasses[self.service_type]

class AreaServiceManager(models.Manager):
    def get_queryset(self):
        return super(AreaServiceManager, self).get_queryset().filter(service_type='AR')

    def create(self, **kwargs):
        kwargs.update({'service_type': 'AR'})
        return super(AreaServiceManager, self).create(**kwargs)

class AreaService(Service):
    objects = AreaServiceManager()
    def save(self, *args, **kwargs):
        self.service_type = 'AR'
        return super(AreaService, self).save(*args, **kwargs)
    class Meta:
        proxy = True

# Manager and Model class definitions omitted for the other two proxy models

This works as intended such that I can create objects for each proxy model transparently by registering them for the admin interface, and queries for Service.objects will return the proper proxy/subclass objects.

But when I try to add a TabularInline to admin.py –

class ServiceInlineAdmin(admin.TabularInline):
    model = Service
    extra = 0

class BuildingAdmin(admin.ModelAdmin):
    inlines = [ServiceInlineAdmin,]

– I get the following error message for the line self.__class__ = subclasses[self.service_type] in models.py (s. above):

__class__   
<class 'services.models.Service'>
args    
()
kwargs  
{}
self    
Error in formatting: RelatedObjectDoesNotExist: Service has no building.
subclasses  
{'AR': <class 'services.models.AreaService'>,
 'CO': <class 'services.models.ConsumptionService'>,
 'OC': <class 'services.models.OccupancyService'>}

I retrieved all objects via the Django shell, and check my database: All Service objects/entries are linked to a building. Commenting out the custom __init__ function solves the issue, and all linked services are displayed for each building (but I obviously lose the ability to properly subclass the Service objects).

Any pointers to the cause of this error are greatly appreciated.

def __init__(self, *args, **kwargs): 
    super(Service, self).__init__(*args, **kwargs) 
    subclasses = {
        'AR' : AreaService,
        'OC' : OccupancyService,
        'CO' : ConsumptionService,
    }
    if self.id:
        self.__class__ = subclasses[self.service_type]