What is this error? I have created a custom module and added Duplicate button. When i click on Duplicate button i got below Traceback.

Server Traceback (most recent call last):
  File "C:\Program Files (x86)\OpenERP 7.0-20141009-011214\server\openerp\addons\web\session.py", line 89, in send
  File "C:\Program Files (x86)\OpenERP 7.0-20141009-011214\server\.\openerp\netsvc.py", line 296, in dispatch_rpc
  File "C:\Program Files (x86)\OpenERP 7.0-20141009-011214\server\.\openerp\service\web_services.py", line 626, in dispatch
  File "C:\Program Files (x86)\OpenERP 7.0-20141009-011214\server\.\openerp\osv\osv.py", line 190, in execute_kw
  File "C:\Program Files (x86)\OpenERP 7.0-20141009-011214\server\.\openerp\osv\osv.py", line 132, in wrapper
  File "C:\Program Files (x86)\OpenERP 7.0-20141009-011214\server\.\openerp\osv\osv.py", line 199, in execute
  File "C:\Program Files (x86)\OpenERP 7.0-20141009-011214\server\.\openerp\osv\osv.py", line 187, in execute_cr
  File "C:\Program Files (x86)\OpenERP 7.0-20141009-011214\server\.\openerp\osv\orm.py", line 5167, in copy
  File "C:\Program Files (x86)\OpenERP 7.0-20141009-011214\server\.\openerp\osv\orm.py", line 5064, in copy_data
  File "C:\Program Files (x86)\OpenERP 7.0-20141009-011214\server\.\openerp\osv\orm.py", line 3703, in read
  File "C:\Program Files (x86)\OpenERP 7.0-20141009-011214\server\.\openerp\osv\orm.py", line 3826, in _read_flat
  File "C:\Program Files (x86)\OpenERP 7.0-20141009-011214\server\.\openerp\osv\fields.py", line 537, in get
AttributeError: 'NoneType' object has no attribute 'search'

What is the issue. How to resolve it?