Error: object code name '' is not valid, while creating user using API,

Hi, I am getting the following error while inserting new user using the API. I am trying to create user by running a task, which will read a csv file and create the user.The object code name '' is not valid. The code name can contain only alphanumeric characters, some special characters (_, -, .) and cannot start or end with '.'.,Could you please let me know how to resolve this.Thanks, Joyanta