code: def clean(self):
          
          cleaned_data = super(EnterpriseCustomerAdminForm, self).clean()
          if 'catalog' in cleaned_data and not cleaned_data['catalog']:
              cleaned_data['catalog'] = None
          return cleaned_data
docs: Clean form fields prior to database entry.
  
          In this case, the major cleaning operation is substituting a None value for a blank
          value in the Catalog field.