In the shell insert a BinData with subtype 2: > db.coll.insert({x: BinData(2, '1234')}) And in the driver you try to read that, you get a parsing exception: > python -i >>> from pymongo import MongoClient >>> c = MongoClient() >>> c.test.test.find_one() Traceback (most recent call last): File "<stdin>", line 1, in <module> File "/Users/kevinalbertson/code/mongo-python-driver/pymongo/collection.py", line 1267, in find_one for result in cursor.limit(-1): File "/Users/kevinalbertson/code/mongo-python-driver/pymongo/cursor.py", line 1200, in next if len(self.__data) or self._refresh(): File "/Users/kevinalbertson/code/mongo-python-driver/pymongo/cursor.py", line 1114, in _refresh self.__send_message(q) File "/Users/kevinalbertson/code/mongo-python-driver/pymongo/cursor.py", line 988, in __send_message codec_options=self.__codec_options) File "/Users/kevinalbertson/code/mongo-python-driver/pymongo/cursor.py", line 1077, in _unpack_response return response.unpack_response(cursor_id, codec_options) File "/Users/kevinalbertson/code/mongo-python-driver/pymongo/message.py", line 1474, in unpack_response return bson.decode_all(self.payload_document, codec_options) bson.errors.InvalidBSON: invalid length or type code