Django loaddata throws ValidationError: [u'Enter a valid date in YYYY-MM-DD format.'] on null=true field?

Question

When I run:

   django-admin.py loaddata ../data/library_authors.json 

the error is:

   ...
   ValidationError: [u'Enter a valid date in YYYY-MM-DD format.']

The model:

   class Writer(models.Model):
     first = models.CharField(u'First Name', max_length=30)
     other = models.CharField(u'Other Names', max_length=30, blank=True)
     last = models.CharField(u'Last Name', max_length=30)
     dob = models.DateField(u'Date of Birth', blank=True, null=True)

     class Meta:
       abstract = True
       ordering = ['last']
       unique_together = ("first", "last")

   class Author(Writer):
     language = models.CharField(max_length=20, choices=LANGUAGES, blank=True)

     class Meta:
       verbose_name = 'Author'
       verbose_name_plural = 'Authors'

Note that the dob DateField has blank=True, null=True

The json file has structure:

[
    {
        "pk": 1, 
        "model": "books.author", 
        "fields": {
            "dob": "", 
            "other": "", 
            "last": "Carey", 
            "language": "", 
            "first": "Peter"
        }
    }, 
    {
        "pk": 3, 
        "model": "books.author", 
        "fields": {
            "dob": "", 
            "other": "", 
            "last": "Brown", 
            "language": "", 
            "first": "Carter"
        }
    }
]

The backing mysql database has the relevent date field in the relevant table set to NULL as default and Null? = YES.

Any ideas on what I'm doing wrong or how I can get loaddata to accept null date values?

===== EDIT:

Daniel's answer worked, I'm redfaced.

Answer 1

Simply change the "" for dob to null and it will work, the empty string isn't the same as null. Even though you have blank=True in there, that's really only for form validation and not input validation through fixtures.