coercing to Unicode: need string or buffer, NoneType found

class Variation(models.Model):
    barcode = models.CharField(max_length=255, unique=True, null=True, blank=True,)
    sku = models.CharField(max_length=255, unique=True, null=False, blank=False)
    title = models.CharField(max_length=255, null=False, unique=False)
    price = models.DecimalField(max_digits=5, decimal_places=2, null=True, blank=True)
    cost = models.DecimalField(max_digits=5, decimal_places=2, null=True, blank=True)
    stock_level = models.IntegerField(null=False, blank=False)
    weight = models.DecimalField(null=True, blank=True, max_digits=5, decimal_places=5)
    height = models.DecimalField(null=True, blank=True, max_digits=5, decimal_places=5)
    width = models.DecimalField(null=True, blank=True, max_digits=5, decimal_places=5)
    length = models.DecimalField(null=True, blank=True, max_digits=5, decimal_places=5)
    is_master = models.BooleanField()
    image_id = models.IntegerField(null=True, blank=True)
    product = models.ForeignKey(Product, related_name='variations') \
        # Reference to Product Model
    created_at = models.DateTimeField(auto_now_add=True)
    updated_at = models.DateTimeField(auto_now=True)

    # 'Magic Method'Helps to view instances of a django model.
    def __unicode__(self):  # __str__ on Python 3
        return self.title

# Property Table called ExtraField
class ExtraField(models.Model):
    '''
    This Model will hold any extra properity or data associated with any
    specific variation.
    '''
    variation = models.ManyToManyField(Variation)  # reference to Variation
    title = models.CharField(max_length=255)
    description = models.CharField(max_length=255)
    created_at = models.DateTimeField(auto_now_add=True)
    updated_at = models.DateTimeField(auto_now=True)

    # 'Magic Method'Helps to view instances of a django model.
    def __unicode__(self):  # __str__ on Python 3
        self self.title