invalid input syntax for type date: "" in django, postgresql?

I am using this code:

def saveUploadedInventory(self, inventory_file,user_id):
        print "Inventory File"
        with open('uploaded_inventory_sheet.csv','wb+') as destination:
            for chunk in inventory_file.chunks():
                destination.write(chunk)
        print "Inventory Saved."
        f = open('uploaded_inventory_sheet.csv','rb')


        with open('uploaded_inventory_sheet.csv') as f:
            next(f)
            content = StringIO('\n'.join(line for line in f))
            self.cur.copy_from(content,'fk_payment_temp', sep=',', null='null', columns=('settlement_ref_no', 'order_type', 'fulfilment_type', 'seller_sku', 
            'wsn', 'order_id', 'order_item_id', 'order_date', 'dispatch_date', 'delivery_date', 
            'cancellation_date', 'settlement_date', 'order_status', 'quantity', 'order_item_value', 
            'sale_transaction_amount', 'discount_transaction_amount', 'refund', 
            'protection_fund', 'total_marketplace_fee', 'service_tax', 'swach_bharat_cess', 
            'settlement_value', 'commission_rate', 'commission', 'payment_rate', 
            'payment_fee', 'fee_discount', 'cancellation_fee', 'fixed_fee', 'emi_fee', 
            'total_weight', 'weight_type', 'shipping_fee', 'reverse_shipping_fee', 
            'shipping_zone', 'token_of_apology', 'pick_and_pack_fee', 'storage_fee', 
            'removal_fee', 'invoice_id', 'invoice_date', 'invoice_amount', 'sub_category', 
            'total_offer_amount', 'my_offer_share', 'flipkart_offer_share'))
        f.close()

It is giving error:

    DataError at /upload/
invalid input syntax for type date: ""
CONTEXT:  COPY fk_payment_temp, line 1, column cancellation_date: ""

there is lots of date column dont have any value.

my table structure is:

CREATE TABLE fk_payment_temp
(
  id serial NOT NULL,
  settlement_ref_no character varying,
  order_type character varying,
  fulfilment_type character varying,
  seller_sku character varying,
  wsn character varying,
  order_id character varying,
  order_item_id character varying,
  order_date timestamp without time zone,
  dispatch_date date default null,
  delivery_date date default null,
  cancellation_date date default null,
  settlement_date date,
  order_status character varying,
 .....

Any body can help me to solve this problem .

either changing python code or changing postgresql structure ?

to converting date to varchar is least option.