When I use empty port body to update port, neutron server return 500

req_sess = requests.session()
req_sess.headers["X-Auth-Token"] = token
resp = req_sess.put("http://10.180.210.69:9696/v2.0/ports/d13ed771-b175-4f22-8a65-4447f72e9bcd", 
 json=dict(port=dict()))

here's the neutron server error:

Jul 30 06:44:41 nfs neutron-server[11250]: ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib64/python2.7/site-packages/sqlalchemy/engine/base.py", line 1248, in _execute_context
Jul 30 06:44:41 nfs neutron-server[11250]: ERROR neutron.pecan_wsgi.hooks.translation     e, statement, parameters, cursor, context
Jul 30 06:44:41 nfs neutron-server[11250]: ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib64/python2.7/site-packages/sqlalchemy/engine/base.py", line 1464, in _handle_dbapi_except
Jul 30 06:44:41 nfs neutron-server[11250]: ERROR neutron.pecan_wsgi.hooks.translation     util.raise_from_cause(newraise, exc_info)
Jul 30 06:44:41 nfs neutron-server[11250]: ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib64/python2.7/site-packages/sqlalchemy/util/compat.py", line 399, in raise_from_cause
Jul 30 06:44:41 nfs neutron-server[11250]: ERROR neutron.pecan_wsgi.hooks.translation     reraise(type(exception), exception, tb=exc_tb, cause=cause)
Jul 30 06:44:41 nfs neutron-server[11250]: ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib64/python2.7/site-packages/sqlalchemy/engine/base.py", line 1244, in _execute_context
Jul 30 06:44:41 nfs neutron-server[11250]: ERROR neutron.pecan_wsgi.hooks.translation     cursor, statement, parameters, context
Jul 30 06:44:41 nfs neutron-server[11250]: ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib64/python2.7/site-packages/sqlalchemy/engine/default.py", line 550, in do_execute
Jul 30 06:44:41 nfs neutron-server[11250]: ERROR neutron.pecan_wsgi.hooks.translation     cursor.execute(statement, parameters)
Jul 30 06:44:41 nfs neutron-server[11250]: ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib/python2.7/site-packages/pymysql/cursors.py", line 170, in execute
Jul 30 06:44:41 nfs neutron-server[11250]: ERROR neutron.pecan_wsgi.hooks.translation     result = self._query(query)
Jul 30 06:44:41 nfs neutron-server[11250]: ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib/python2.7/site-packages/pymysql/cursors.py", line 328, in _query
Jul 30 06:44:41 nfs neutron-server[11250]: ERROR neutron.pecan_wsgi.hooks.translation     conn.query(q)
Jul 30 06:44:41 nfs neutron-server[11250]: ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib/python2.7/site-packages/pymysql/connections.py", line 517, in query
Jul 30 06:44:41 nfs neutron-server[11250]: ERROR neutron.pecan_wsgi.hooks.translation     self._affected_rows = self._read_query_result(unbuffered=unbuffered)
Jul 30 06:44:41 nfs neutron-server[11250]: ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib/python2.7/site-packages/pymysql/connections.py", line 732, in _read_query_result
Jul 30 06:44:41 nfs neutron-server[11250]: ERROR neutron.pecan_wsgi.hooks.translation     result.read()
Jul 30 06:44:41 nfs neutron-server[11250]: ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib/python2.7/site-packages/pymysql/connections.py", line 1075, in read
Jul 30 06:44:41 nfs neutron-server[11250]: ERROR neutron.pecan_wsgi.hooks.translation     first_packet = self.connection._read_packet()
Jul 30 06:44:41 nfs neutron-server[11250]: ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib/python2.7/site-packages/pymysql/connections.py", line 684, in _read_packet
Jul 30 06:44:41 nfs neutron-server[11250]: ERROR neutron.pecan_wsgi.hooks.translation     packet.check_error()
Jul 30 06:44:41 nfs neutron-server[11250]: ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib/python2.7/site-packages/pymysql/protocol.py", line 220, in check_error
Jul 30 06:44:41 nfs neutron-server[11250]: ERROR neutron.pecan_wsgi.hooks.translation     err.raise_mysql_exception(self._data)
Jul 30 06:44:41 nfs neutron-server[11250]: ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib/python2.7/site-packages/pymysql/err.py", line 109, in raise_mysql_exception
Jul 30 06:44:41 nfs neutron-server[11250]: ERROR neutron.pecan_wsgi.hooks.translation     raise errorclass(errno, errval)
Jul 30 06:44:41 nfs neutron-server[11250]: ERROR neutron.pecan_wsgi.hooks.translation DBError: (pymysql.err.ProgrammingError) (1064, u"You have an error in your SQL syntax; check the manual t
Jul 30 06:44:41 nfs neutron-server[11250]: ERROR neutron.pecan_wsgi.hooks.translation [SQL: UPDATE ml2_port_bindings SET vnic_type=%(vnic_type)s WHERE ml2_port_bindings.port_id = %(ml2_port_b
Jul 30 06:44:41 nfs neutron-server[11250]: ERROR neutron.pecan_wsgi.hooks.translation [parameters: {'vnic_type': {}, 'ml2_port_bindings_host': u'', 'ml2_port_bindings_port_id': u'd13ed771-b17
Jul 30 06:44:41 nfs neutron-server[11250]: ERROR neutron.pecan_wsgi.hooks.translation (Background on this error at: http://sqlalche.me/e/f405)
Jul 30 06:44:41 nfs neutron-server[11250]: ERROR neutron.pecan_wsgi.hooks.translation