KeyError: u'max_digits' with django decimal field

Question

I have the model with below fields at starting

class Option(models.Model):
    description = models.CharField(max_length = 100, null = False, blank = False)
    price = models.PositiveIntegerField(null = False, blank = False, editable = True)

At first, i designed the model with price field as PositiveIntegerField, but when we enter a decimal value like 5.36, 56.2314 etc., i was getting a Keyerror : 'invalid'

So i decided to change the field from PositiveIntegerField to DecimalField and change the price field like below

price = models.DecimalField(max_digits = 6, decimal_places = 2, null = False, blank = False)  # price in dollars

But when we tried to enter the decimal values like 32.56, 5.36, etc., and when tried to save the record, it was throwing me an error like below

u'max_digits'
Request Method: POST
Request URL:    http://127.0.0.1:8000/option/new/192/
Django Version: 1.5.4
Exception Type: KeyError
Exception Value:    
u'max_digits'
Exception Location: /home/user/Envs/proj/local/lib/python2.7/site-packages/django/forms/fields.py in validate, line 320

and error in views.py at line 23

21 if request.method == 'POST':
22        option_formset = OptionFormset(request.POST, request.FILES)
23        if option_formset.is_valid(): 

So why am i facing the keyerror when i changed the price from PositiveIntegerField to DecimalField ?, how to avoid this, any extra work need to be done, to change the field (if it already was a different type)

forms.py

class OptionForm(ModelForm):
    class Meta:
        model = Option

OptionFormset = formset_factory(OptionForm, max_num=10, formset=RequiredFormSet)