mongo查询抛出异常，什么原因？

软件版本：python2.7，pymongo3.2 ,MongoDB3.2
错误信息:在用find_one方法查询时 data=db.find_one({'cid':tempDict['cid']}) ，抛出如下错, 查了好久也没查出原因。
File "build/bdist.linux-x86_64/egg/pymongo/collection.py", line 1006, in find_one

for result in cursor.limit(-1):

File "build/bdist.linux-x86_64/egg/pymongo/cursor.py", line 1097, in next

if len(self.__data) or self._refresh():

File "build/bdist.linux-x86_64/egg/pymongo/cursor.py", line 1019, in _refresh

self.__read_concern))

File "build/bdist.linux-x86_64/egg/pymongo/cursor.py", line 905, in __send_message

helpers._check_command_response(doc['data'][0])

File "build/bdist.linux-x86_64/egg/pymongo/helpers.py", line 196, in _check_command_response

raise OperationFailure(msg % errmsg, code, response)

OperationFailure: could not find host matching read preference { mode: "primary", tags: [ {} ] } for set rs1