Django InlineFormset ValueError

models.py

from django.db import models

class Container(models.Model):
	pass

class Item(models.Model):
	container = models.ForeignKey(Container)
	data = models.CharField(max_length=123)

test.py

from django.test import TestCase
from django.forms.models import inlineformset_factory, BaseInlineFormSet
from test_app.models import Container, Item

TestFormSet = inlineformset_factory(Container, Item, extra=0)

class MyTestCase(TestCase):
	def setUp(self):
		self.container = Container.objects.create()
		self.existingItem = Item.objects.create(container=self.container, data="asdf")
		self.test_data = {
			"item_set-TOTAL_FORMS": "2",
			"item_set-MAX_NUM_FORMS": "1000",
			"item_set-INITIAL_FORMS": "1",
			
			"item_set-0-id": "", # Not valid
			"item_set-0-data": "foo",
			
			"item_set-1-id": str(self.existingItem.id),
			"item_set-1-data": str(self.existingItem.data),
		}
	
	def test_django_formset_int_error(self):
		formset = TestFormSet(self.test_data, instance=self.container)
		try:
			formset.is_valid()
		except ValueError:
			self.fail("Formset.is_valid() raised unexpected exception.")