Django modelform not saving foreignkey

I have the following code which is supposed to create a MyConfig object. However, it doesn't as the app_model is always returned as None.

The idea is to choose from a select few contenttypes and then add a key, and the resulting config will trigger a bunch of services. However whenever I save the form, the contenttype stored in the app_model is always None, which is clearly undesirable.

This is in Django1.8

Here is the admin:

class ContentTypeModelChoiceField(ModelChoiceField):      def label_from_instance(self, obj):          return "{}-{}".format(obj.app_label, obj.model)      class MyConfigForm(ModelForm):      class Meta:          model = models.MyConfig          fields = '__all__'        def __init__(self, *args, **kwargs):          super(MyConfigForm, self).__init__(*args, **kwargs)          self.fields['app_model'].label = "App Name"        app_model = ContentTypeModelChoiceField(          ContentType.objects.filter(              app_label__startswith='myApp',              model="myModel",          ),          empty_label="Choose a model",      )      class MyConfigAdmin(admin.ModelAdmin):      model = models.MyConfig      form =  MyConfigForm      list_display = (<display fields>      )      search_fields = (<search fields>      )

And here is the model itself:

class MyConfig(models.Model):        app_model = models.ForeignKey(ContentType, null=True)      ref_key = models.CharField(max_length=32, null=True)

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