Error in GTK-client in some forms using on_change

in GTK Openerp Client: click to "FLEET MANAGER ->Operations Managment->Fuel Log" and "FLEET MANAGER ->Operations Managment->Fleet Card Expenses",getting error:

File "/home/admin/Dropbox/workspace/OpenERP/src/OpenERP/6.1/site-packages/openerp/wsgi/core.py", line 80, in xmlrpc_return
    response = xmlrpclib.dumps((result,), methodresponse=1, allow_none=False, encoding=None)
  File "/usr/lib/python2.7/xmlrpclib.py", line 1085, in dumps
    data = m.dumps(params)
  File "/usr/lib/python2.7/xmlrpclib.py", line 632, in dumps
    dump(v, write)
  File "/usr/lib/python2.7/xmlrpclib.py", line 654, in __dump
    f(self, value, write)
  File "/usr/lib/python2.7/xmlrpclib.py", line 735, in dump_struct
    dump(v, write)
  File "/usr/lib/python2.7/xmlrpclib.py", line 654, in __dump
    f(self, value, write)
  File "/usr/lib/python2.7/xmlrpclib.py", line 735, in dump_struct
    dump(v, write)
  File "/usr/lib/python2.7/xmlrpclib.py", line 654, in __dump
    f(self, value, write)
  File "/usr/lib/python2.7/xmlrpclib.py", line 658, in dump_nil
    raise TypeError, "cannot marshal None unless allow_none is enabled"
TypeError: cannot marshal None unless allow_none is enabled

This is a GTK Client error only, as https://answers.launchpad.net
/openobject-server/+question/201487