I have this code:

import time
from openerp.osv import fields, osv
import openerp.addons.decimal_precision as dp
from openerp.tools.translate import _    
from openerp.tools import hashlib       
from datetime import datetime          
from operator import itemgetter      
from openerp import SUPERUSER_ID
from openerp import pooler, tools
from dateutil.relativedelta import relativedelta

class account_bank_payment(osv.osv):
    _name = "account.bank.payment"
    _description = 'PayDirect Tax Payments'
    _inherit = 'account.bank.statement.line'
    _columns = {        
        'name': fields.char('Payment Reference', size=255, required=True),
        'date': fields.date('Payment Date', required=True),
        'amount': fields.float('Amount', digits_compute=dp.get_precision('Account'), required=True),                                                 
        'ref': fields.char('Description', size=255, required=True),    
        'taxpayer_name': fields.char('Tax Payer', size=255, required=True),
        'head_subhead': fields.char('Head/Sub-head', size=255, required=True), 
        'instr_number': fields.char('Print Paper Number', size=20), 
        'system_print_number': fields.char('System Print Number', size=20), 
        'bank_name': fields.char('Bank Name', size=255, required=True),     
        'payment_type': fields.char('Payment Type', size=255, required=True),
        'payment_logid': fields.integer('Payment Log Id'),                 
        'terminal_id': fields.char('Terminal Id', size=255),                 
        'channel_name': fields.char('Channel Name', size=255),
        'location': fields.char('Channel Name', size=255),                  
        'institution_id': fields.char('Institution Id', size=255),         
        'institution_name': fields.char('Institution', size=255),
        'branch_name': fields.char('Branch', size=255),   
        'receipt_no': fields.char('Receipt No', size=255),  
        'collections_account': fields.char('Collections Account', size=255),                                                          
        'bank_code': fields.char('Bank Code', size=255),   
        'is_reversal': fields.boolean('Is Reversal', help="Indicates whether the transactions is a reversal or not."),                                                          
        'deposit_slip_number': fields.char('Deposit Slip Number', size=255),              
        'period_covered': fields.char('Period Covered', size=255),  
        'period_covered_start': fields.date('Period Covered Start'),                                      
        'period_covered_end': fields.date('Period Covered End'),
        'val_date': fields.date('Value Date'),  
        'cust_reference': fields.char('Customer Reference', size=255),
        'alternate_cust_reference': fields.char('Alternate Customer Reference', size=255),
        'payment_status': fields.integer('Payment Status'),
        'settlement_date': fields.date('Settlement Date'),
        'fee_name': fields.char('Fee Name', size=255),
        'othercustomer_info': fields.char('Other Customer Info', size=255),
        'collections_account': fields.char('Collections Account', size=255),
        'third_party_code': fields.char('Thirty Party Code', size=255),
        'customer_address': fields.char('Customer Address', size=255),
        'customer_phone_number': fields.char('Customer Phone Number', size=255),
        'depositor_name': fields.char('Depositor Name', size=255),
        'payment_currency': fields.char('Payment Currency', size=255),
        'product_group_code': fields.char('Product Group Code', size=255),
        'transaction_date': fields.date('Transaction Date'),
        'item_amount': fields.float('Item Amount', digits_compute=dp.get_precision('Account')),
        'lead_bank_code': fields.char('Lead Bank Code', size=255),
        'lead_bank_cbn_code': fields.char('Lead Bank CBN Code', size=255),
        'lead_bank_name': fields.char('Lead Bank Name', size=255),
        'ip': fields.char('IP', size=255), 
        #dummy fields for date range
        'payment_date_from':fields.function(lambda *a,**k:{}, method=True, type='date',string="Payment date from"),
        'payment_date_to':fields.function(lambda *a,**k:{}, method=True, type='date',string="Payment date to"),           
        #'status': fields.selection([('ready', 'Ready'), ('printed', 'Printed'), ('issued', 'Issued'), ('request', 'Void Request')], 'State'),   
        'state': fields.selection([('ready', 'Ready'), ('printed', 'Printed'), ('issued', 'Issued'), ('request', 'Void Request'), ('voided', 'Voided')], 'Status'),    
        'wht_schedule_id': fields.one2many('collections.wht.schedule', 'payment_reference', 'WHT Schedule', select=False, required=False, ondelete='cascade'),       
        'partner_id': fields.many2one('res.partner', 'Internal Payer Name'),
        #override the following here to remove required constraint    
        'statement_id': fields.many2one('account.bank.statement', 'Statement',
            select=False, required=False, ondelete='cascade'),
        'type': fields.selection([ 
            ('taxpayer','Tax Payer'),
            ('supplier','Supplier'),
            ('customer','Customer'),
            ('general','General')
            ], 'Type', required=False),
        'account_id': fields.many2one('account.account','Account',
            required=False),          
    }
    _defaults = {
        #'status': 'ready', 
        'state': 'ready',
        'period_covered_start' : lambda *a : None, 
        'period_covered_end' : lambda *a : None,
    }
    _sql_constraints = [
#         ('name_unique', 'unique(name)', 'The payment reference must be unique per payment!'),
#         ('payment_logid_unique', 'unique(payment_logid)', 'The payment log id must be unique per payment!'),
#         ('terminal_id', 'unique(terminal_id)', 'The terminal id must be unique per payment!'), 
         ('instr_number', 'unique(instr_number)', 'The print paper number must be unique for each payment!'),
         ('system_print_number', 'unique(system_print_number)', 'The system print number must be unique for each payment!'),
    ]
    _order = "date desc"

    def unlink(self, cr, uid, ids, context=None):
        if context is None:
            context = {}
#         if context.get('block_statement_line_delete', False):
#             raise osv.except_osv(_('Warning!'), _('Delete operation not allowed. \
#             Please go to the associated bank statement in order to delete and/or modify bank statement line.'))
        return super(account_bank_payment, self).unlink(cr, uid, ids, context=context)

    def set_ready(self, cr, uid, ids, context):
        self.write(cr, uid, ids, {'state': 'ready'}, context=context)
        return {}

    def set_printed(self, cr, uid, ids, context):
        for payment in self.browse(cr, uid, ids, context): 
            if not payment.instr_number:
                raise osv.except_osv(_('Warning!'), _('You must enter the print paper number before setting the status to printed.'))
            else:
                self.write(cr, uid, ids, {'state': 'printed'}, context=context)
        return {}

    def set_issued(self, cr, uid, ids, context):
        self.write(cr, uid, ids, {'state': 'issued'}, context=context)
        return {} 

    def set_request(self, cr, uid, ids, context):
        for payment in self.browse(cr, uid, ids, context): 
            if not payment.note:
                raise osv.except_osv(_('Warning!'), _('You must add a note to this payment before requesting it to be voided.'))
            else:
                self.write(cr, uid, ids, {'state': 'request'}, context=context)
        return {}   

    def set_voided(self, cr, uid, ids, context):
        self.write(cr, uid, ids, {'state': 'voided'}, context=context)
        return {} 

    def onchange_papernumber(self, cr, uid, ids, instr_number, context=None):
        if instr_number:
          hash = hashlib.sha1()
          hash.update(instr_number)
          self.write(cr, uid, ids, {'system_print_number': '89' + hash.hexdigest()[:12].upper()}, context=context)               
        return {}  

    def create(self, cr, uid, vals, context=None):         
#         if vals['period_covered_start'] and vals['period_covered_end']: 
#             if vals['period_covered_end'] < vals['period_covered_start']:
#                 raise osv.except_osv(_('Warning!'), _('End date cannot be earlier than start date.')) 
#         result = super(account_bank_payment,self).create(cr, uid, vals, context=context)             
        return result  

    def write(self, cr, uid, ids, vals, context=None):
      if uid != 15 and vals['head_subhead']:   
          raise osv.except_osv(_('Warning!'), _('End date cannot be earlier than start date.'))
#         if vals['period_covered_start'] and vals['period_covered_end']: 
#             if vals['period_covered_end'] < vals['period_covered_start']:
#                 raise osv.except_osv(_('Warning!'), _('End date cannot be earlier than start date.'))  
#         result = super(account_bank_payment,self).write(cr, uid, ids, vals, context=context)             
      return result  

    def onchange_period_covered(self, cr, uid, ids, period_covered_start, period_covered_end, context=None):
        if period_covered_start and period_covered_end: 
            if period_covered_end < period_covered_start:
                raise osv.except_osv(_('Warning!'), _('End date cannot be earlier than start date.'))               
        return {}

    def onchange_head(self, cr, uid, ids, context=None):
      if uid != 15:   
          raise osv.except_osv(_('Error!'), _('You are not allowed to edit this field.'))               
      return {}  

    def onchange_description(self, cr, uid, ids, context=None):
      if uid != 15:   
          raise osv.except_osv(_('Error!'), _('You are not allowed to edit this field.'))               
      return {}

account_bank_payment()


class paydirect_payment(osv.osv):
    _name = "payments"
    _description = 'PayDirect Raw Payment Records'
    _columns = {        
        'paymentreference': fields.char('Payment Reference', size=255),
        'paymentdate': fields.date('Payment Date'),
        'itemamount': fields.float('Amount', digits_compute=dp.get_precision('Account')),                                                 
        'itemname': fields.char('Description', size=255),    
        'customername': fields.char('Customer Name', size=255),
        'itemcode': fields.char('Head/Sub-head', size=255),  
        'bankname': fields.char('Bank Name', size=255),     
        'payment_type': fields.char('Payment Type', size=255),
        'paymentlogid': fields.integer('Payment Log Id'),                 
        'terminalid': fields.char('Terminal Id', size=255),                 
        'channelname': fields.char('Channel Name', size=255),
        'location': fields.char('Channel Name', size=255),                  
        'institutionid': fields.char('Institution Id', size=255),         
        'institutionname': fields.char('Institution', size=255),
        'branchname': fields.char('Branch', size=255),   
        'receiptno': fields.char('Receipt No', size=255),  
        'collectionsaccount': fields.char('Collections Account', size=255),                                                          
        'bankcode': fields.char('Bank Code', size=255),   
        'isreversal': fields.boolean('Is Reversal', help="Indicates whether the transactions is a reversal or not."),                                                          
        'depositslipnumber': fields.char('Deposit Slip Number', size=255),                
        'custreference': fields.char('Customer Reference', size=255),
        'alternatecustreference': fields.char('Alternate Customer Reference', size=255),
        'paymentstatus': fields.integer('Payment Status'),
        'settlementdate': fields.date('Settlement Date'),
        'feename': fields.char('Fee Name', size=255),
        'othercustomerinfo': fields.char('Other Customer Info', size=255),
        'collectionsaccount': fields.char('Collections Account', size=255),
        'thirdpartycode': fields.char('Thirty Party Code', size=255),
        'customeraddress': fields.char('Customer Address', size=255),
        'customerphonenumber': fields.char('Customer Phone Number', size=255),
        'depositorname': fields.char('Depositor Name', size=255),
        'paymentcurrency': fields.char('Payment Currency', size=255),
        'productgroupcode': fields.char('Product Group Code', size=255),
        'transactiondate': fields.date('Transaction Date'),
        'itemamount': fields.float('Item Amount', digits_compute=dp.get_precision('Account')),
        'leadbankcode': fields.char('Lead Bank Code', size=255),
        'leadbankcbncode': fields.char('Lead Bank CBN Code', size=255),
        'leadbankname': fields.char('Lead Bank Name', size=255),
        'ip': fields.char('IP', size=255),          
    }
    _order = "paymentdate desc"

paydirect_payment()

# vim:expandtab:smartindent:tabstop=4:softtabstop=4:shiftwidth=4:

When I try to edit a record and save, I get this error:

    Client Traceback (most recent call last):
  File "C:\Program Files (x86)\OpenERP 7.0\Server\server\openerp\addons\web\http.py", line 204, in dispatch
  File "C:\Program Files (x86)\OpenERP 7.0\Server\server\openerp\addons\web\controllers\main.py", line 1125, in call_kw
  File "C:\Program Files (x86)\OpenERP 7.0\Server\server\openerp\addons\web\controllers\main.py", line 1117, in _call_kw
  File "C:\Program Files (x86)\OpenERP 7.0\Server\server\openerp\addons\web\session.py", line 42, in proxy
  File "C:\Program Files (x86)\OpenERP 7.0\Server\server\openerp\addons\web\session.py", line 30, in proxy_method
  File "C:\Program Files (x86)\OpenERP 7.0\Server\server\openerp\addons\web\session.py", line 103, in send


Server Traceback (most recent call last):
  File "C:\Program Files (x86)\OpenERP 7.0\Server\server\openerp\addons\web\session.py", line 89, in send
  File "C:\Program Files (x86)\OpenERP 7.0\Server\server\.\openerp\netsvc.py", line 292, in dispatch_rpc
  File "C:\Program Files (x86)\OpenERP 7.0\Server\server\.\openerp\service\web_services.py", line 626, in dispatch
  File "C:\Program Files (x86)\OpenERP 7.0\Server\server\.\openerp\osv\osv.py", line 188, in execute_kw
  File "C:\Program Files (x86)\OpenERP 7.0\Server\server\.\openerp\osv\osv.py", line 131, in wrapper
  File "C:\Program Files (x86)\OpenERP 7.0\Server\server\.\openerp\osv\osv.py", line 197, in execute
  File "C:\Program Files (x86)\OpenERP 7.0\Server\server\.\openerp\osv\osv.py", line 185, in execute_cr
  File "C:\Program Files (x86)\OpenERP 7.0\Server\server\openerp\addons\account_collections\account_bank_payment.py", line 169, in write
KeyError: 'head_subhead'

Can someone help identify my error?

Thanks in advance.