Load Django fixtures with nullable foreign key fields

I made a set of fixtures with two nullable foreign key fields, but for some reason when I try loaddata I get complaints that the fields have to be integers.

model (deleted some stuff for brevity):

class Network(models.Model):
    network     = models.GenericIPAddressField()
    mask        = models.CharField(max_length=2)
    description = models.CharField(max_length=2000)
    designation = models.CharField(choices=DESIGNATION_CHOICES, max_length=10)
    site        = models.CharField(choices=SITE_CHOICES, max_length=5)
    category    = models.CharField(choices=CATEGORY_CHOICES, max_length=10)
    vlan        = models.CharField(max_length=5, blank=True, null=True)
    vrf         = models.ForeignKey(Vrf,on_delete=models.CASCADE, blank=True, null=True)
    parent      = models.ForeignKey('self', on_delete=models.CASCADE, related_name='supernet', blank=True, null=True)

fixture block:

<django-objects version="1.0">
    <object model="ipmanager.network" pk="459124212">
        <field name="mask" type="CharField">30</field>
        <field name="site" type="CharField">place</field>
        <field name="network" type="CharField">1.1.1.1</field>
        <field name="category" type="CharField">category</field>
        <field name="description" type="CharField">enum amun set ra</field>
        <field name="vlan" type="CharField" />
        <field name="designation" type="CharField">supernet</field>
        <field name="parent" rel="ManyToOneRel" to="networks.id"></field>
        <field name="vrf" rel="ManyToOneRel" to="vrfs.id"></field>
    </object>

I've tried Null, None, empty string, and a space and django won't accept any of them. Here's the error I get:

>python manage.py loaddata ipmanager/net
works_fixture.xml
Traceback (most recent call last):
  File "C:\Users\dchadwell.CORP\AppData\Local\Programs\Python\Python35\lib\site-
packages\django\db\models\fields\__init__.py", line 955, in to_python
    return int(value)
ValueError: invalid literal for int() with base 10: ''

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "manage.py", line 10, in <module>
    execute_from_command_line(sys.argv)
  File                 "C:\Users\dchadwell.CORP\AppData\Local\Programs\Python\Python35\lib\site-
packages\django\core\management\__init__.py", line 353, in     execute_from_command_
line
    utility.execute()
  File     "C:\Users\dchadwell.CORP\AppData\Local\Programs\Python\Python35\lib\site-
packages\django\core\management\__init__.py", line 345, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
File     "C:\Users\dchadwell.CORP\AppData\Local\Programs\Python\Python35\lib\site-
packages\django\core\management\base.py", line 348, in run_from_argv
    self.execute(*args, **cmd_options)
  File     "C:\Users\dchadwell.CORP\AppData\Local\Programs\Python\Python35\lib\site-
packages\django\core\management\base.py", line 399, in execute
    output = self.handle(*args, **options)
  File     "C:\Users\dchadwell.CORP\AppData\Local\Programs\Python\Python35\lib\site-
packages\django\core\management\commands\loaddata.py", line 60, in handle
    self.loaddata(fixture_labels)
  File     "C:\Users\dchadwell.CORP\AppData\Local\Programs\Python\Python35\lib\site-
packages\django\core\management\commands\loaddata.py", line 100, in loaddata
    self.load_label(fixture_label)
  File     "C:\Users\dchadwell.CORP\AppData\Local\Programs\Python\Python35\lib\site-
packages\django\core\management\commands\loaddata.py", line 152, in load_label
    for obj in objects:
  File     "C:\Users\dchadwell.CORP\AppData\Local\Programs\Python\Python35\lib\site-
packages\django\core\serializers\xml_serializer.py", line 177, in __next__
    return self._handle_object(node)
  File     "C:\Users\dchadwell.CORP\AppData\Local\Programs\Python\Python35\lib\site-
packages\django\core\serializers\xml_serializer.py", line 218, in     _handle_object

    data[field.attname] = self._handle_fk_field_node(field_node, field)
  File     "C:\Users\dchadwell.CORP\AppData\Local\Programs\Python\Python35\lib\site-
packages\django\core\serializers\xml_serializer.py", line 258, in _han    dle_fk_fie
ld_node
    return     model._meta.get_field(field.remote_field.field_name).to_python(field_
value)
  File     "C:\Users\dchadwell.CORP\AppData\Local\Programs\Python\Python35\lib\site-
packages\django\db\models\fields\__init__.py", line 960, in to_python
    params={'value': value},
django.core.exceptions.ValidationError: ["'' value must be an integer."]

What am I doing wrong here?

Remove the foreign keys which you don't need, rather than trying to find the empty value, i.e. your fixture block should look something like this instead:

<django-objects version="1.0">
    <object model="ipmanager.network" pk="459124212">
        <field name="mask" type="CharField">30</field>
        <field name="site" type="CharField">place</field>
        <field name="network" type="CharField">1.1.1.1</field>
        <field name="category" type="CharField">category</field>
        <field name="description" type="CharField">enum amun set ra</field>
        <field name="vlan" type="CharField" />
        <field name="designation" type="CharField">supernet</field>
    </object>