cast to objectid failed for value object object at path -- mongoose

[mahesh tanneeru]

i am new to NodeJS and MongoDB and ran through following problem while creating a sample app

   // ************  this is  testusers.js file  ************//

    var userdetails = require('userdetails');

    var mongoose = require('mongoose');

    var schema = mongoose.Schema;

    

    var geekDetails = new schema(

    {

        description:String,

        details:{

            type:mongoose.Schema.Types.ObjectId,

            ref:'userDetails'

        }

    });

    module.exports={

    model:mongoose.model('geekDetails',geekDetails),

    schema:geekDetails

    };

    // ************  this is  user.js file  ************//

    

    var mongoose = require('mongoose');

    var schema = mongoose.Schema;

    var userDetails = new schema({

    firstName :String,

    lastName : String,

    email:String,

    password:String

    });

    module.exports={

    model:mongoose.model('userDetails',userDetails),

    schema:userDetails

    };

     

    // ************  this is  create.js file [used to save the details to DB]  *******//

   

    var testusers = require('testusers.js');

    var mongoose = require('mongoose');

    

    adduser= function(firstName,lastName,email,password,passwordConfirmation,

                      description,callback){

                   new testusers.model({

                description:description,

                details:{

                    firstName:firstName,

                    lastName:lastName,

                    email:email,

                    password:password

                }

            }).save(function(err,result){

                if(err){callback(err,null);}

                    else{callback(null,result);}

        });

when i tried to save the details it returned the following error "**cast to objectid failed for value object object at path 'details'** " i am sure i am doing something wrong with schemas but i couldn't figure it out.

thanks in advance