ValidateError', 'The value "error" for the field "state" is

I keep getting the following error when I try click Process to process an agreement:
Exception in thread Thread-71:
Traceback (most recent call last):
File "/opt/local/Library/Frameworks/Python.framework/Versions/2.5/lib/python2.5/threading.py", line 486, in __bootstrap_inner
self.run()
File "/opt/local/Library/Frameworks/Python.framework/Versions/2.5/lib/python2.5/threading.py", line 663, in run
self.function(*self.args, **self.kwargs)
File "/opt/local/Library/Frameworks/Python.framework/Versions/2.5/lib/python2.5/site-packages/openerp-server/addons/base/ir/ir_cron.py", line 97, in _poolJobs
self._callback(cr, job['user_id'], job['model'], job['function'], job['args'])
File "/opt/local/Library/Frameworks/Python.framework/Versions/2.5/lib/python2.5/site-packages/openerp-server/addons/base/ir/ir_cron.py", line 73, in _callback
f(cr, uid, *args)
File "/opt/local/Library/Frameworks/Python.framework/Versions/2.5/lib/python2.5/site-packages/openerp-server/addons/account_invoicing/invoicing.py", line 479, in _run_filters
agr.write(cr, uid, [agr_id], {'state':'error'})
File "/opt/local/Library/Frameworks/Python.framework/Versions/2.5/lib/python2.5/site-packages/openerp-server/addons/account_invoicing/invoicing.py", line 1878, in write
return super(agreement, self).write(cr, uid, ids, vals, context=context)
File "/opt/local/lib/python2.5/site-packages/openerp-server/osv/orm.py", line 2435, in write
% (vals[field], field))
except_orm: ('ValidateError', 'The value "error" for the field "state" is not in the selection')
Exception in thread Thread-71:
Traceback (most recent call last):
File "/opt/local/Library/Frameworks/Python.framework/Versions/2.5/lib/python2.5/threading.py", line 486, in __bootstrap_inner
self.run()
File "/opt/local/Library/Frameworks/Python.framework/Versions/2.5/lib/python2.5/threading.py", line 663, in run
self.function(*self.args, **self.kwargs)
File "/opt/local/Library/Frameworks/Python.framework/Versions/2.5/lib/python2.5/site-packages/openerp-server/addons/base/ir/ir_cron.py", line 97, in _poolJobs
self._callback(cr, job['user_id'], job['model'], job['function'], job['args'])
File "/opt/local/Library/Frameworks/Python.framework/Versions/2.5/lib/python2.5/site-packages/openerp-server/addons/base/ir/ir_cron.py", line 73, in _callback
f(cr, uid, *args)
File "/opt/local/Library/Frameworks/Python.framework/Versions/2.5/lib/python2.5/site-packages/openerp-server/addons/account_invoicing/invoicing.py", line 479, in _run_filters
agr.write(cr, uid, [agr_id], {'state':'error'})
File "/opt/local/Library/Frameworks/Python.framework/Versions/2.5/lib/python2.5/site-packages/openerp-server/addons/account_invoicing/invoicing.py", line 1878, in write
return super(agreement, self).write(cr, uid, ids, vals, context=context)
File "/opt/local/lib/python2.5/site-packages/openerp-server/osv/orm.py", line 2435, in write
% (vals[field], field))
except_orm: ('ValidateError', 'The value "error" for the field "state" is not in the selection')