I call mongo_insert with vaild parameters and the data is inserted into the database, however it returns -1, and prompts to look at lasterrstr, which is empty. I traced it to mongo_check_last_error which is called if you have a write_concern. Line 770 mongo.c call bson_find looking for err or $err, in the keys of the response. It isn't found however the last line of bson_find returns something.