Django 1.5 AbstractBaseUser with char primary key not JSON serializable

[Kaloian]

I am having the following custom user model trying to use the Django 1.5 
AbstractBaseUser:

class Merchant(AbstractBaseUser): 
    email = models.EmailField()
    company_name = models.CharField(max_length=256)
    website = models.URLField()
    description = models.TextField(blank=True)
    api_key = models.CharField(blank=True, max_length=256, primary_key=True)   

    USERNAME_FIELD = 'email' 
    REQUIRED_FIELDS = ['company_name','website']


   class Meta:
        verbose_name = _('Merchant')
        verbose_name_plural = _('Merchants')

   def __unicode__(self):
        return self.company_name 


The model works perfectly and database is as expected, but the problem is 
when I try to dumpdata to create fixtures for my tests. 

python manage.py dumpdata --natural --exclude=contenttypes 
--exclude=auth.permission --indent=4 > fixtures/initial_data.json


Then I get the error:

CommandError: Unable to serialize database: <Merchant: Test Shop> is not JSON 
serializable



Do you have ideas what could be the reason for this. Could it be the charfield 
primary key or something with the abstractbaseuser model?
  
Thanks

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