How to get rid of the error that occurs when saving the formset?

Question

Django

Error occurs when saving formsets
ValidationError [u"'' value must be an integer."]
Below code:
models.py

class RoyaltiesAd(models.Model):
    isbn = models.CharField(max_length=45, blank=True, null=True)
    supplier_num = models.CharField(max_length=45, blank=True, null=True)
    advance_num = models.IntegerField(blank=True, null=True)
    home_currency_amount = models.DecimalField(max_digits=10, decimal_places=0, blank=True, null=True)
    currency_amount = models.DecimalField(max_digits=10, decimal_places=2, blank=True, null=True)
    pay_question = models.IntegerField(blank=True, null=True)
    invoice_rec_date = models.DateField(blank=True, null=True)
    advance_note = models.CharField(max_length=255, blank=True, null=True)

forms.py

class RoyaltiesAdForm(forms.ModelForm):
    class Meta:
        model = RoyaltiesAd
        fields = '__all__'

views.py

if request.method == 'POST':
    formset = RoyaltiesAdSet(request.POST)
    if formset.is_valid():
        formset.save()
else:
    formset = RoyaltiesAdSet()

return render(request, 'template.html', {'formset': formset,})

template.html

<form method="POST" action="" enctype="multipart/form-data">
      {% csrf_token %}
      {{ formset }}
    </form>

As I understand from the debug, Jang cannot create an id. Who faced such error? Or any ideas how to fix it? Thanks in advance