This field is required error in django

In the model I set:

class Task(models.Model):
    EstimateEffort = models.PositiveIntegerField('Estimate hours',max_length=200)
    Finished = models.IntegerField('Finished percentage',blank=True)

But in the web page, if I didn't set a value for the Finished field, it is showing an error This field is required. I tried null=True and blank=True. But none of them worked. So could you please tell me how can I make a field allowed to be empty.

I have found that there is a attribute empty_strings_allowed, i set it to True, but still the same, and i subclass the models.IntegerField. It still can not work

class IntegerNullField(models.IntegerField):
    description = "Stores NULL but returns empty string"
    empty_strings_allowed =True
    log.getlog().debug("asas")
    def to_python(self, value):
        log.getlog().debug("asas")
        # this may be the value right out of the db, or an instance
        if isinstance(value, models.IntegerField):
            # if an instance, return the instance
            return value
        if value == None:
            # if db has NULL (==None in Python), return empty string
            return ""
        try:
            return int(value)
        except (TypeError, ValueError):
            msg = self.error_messages['invalid'] % str(value)
            raise exceptions.ValidationError(msg)

    def get_prep_value(self, value):
        # catches value right before sending to db
        if value == "":
            # if Django tries to save an empty string, send to db None (NULL)
            return None
        else:
            return int(value) # otherwise, just pass the value