data is not getting saved in mongoose database

I am trying to save my mongoDB with mongoose.app.post function calls the saveLogInfo function. The problem in saveLogInfo function is the code till consoel.log ("inside savae") is runnig but after that I got an error that string is not a function
callback in not getting called and show an error that "saving failed."

the schema is: var LoginInfoSchema=new Schema({

username:String,
password:String

});

app.post function:

var LoginInfo=db.model('LoginInfo',LoginInfoSchema);

function saveLogInfo (username , password ,callback){
console.log("saveLogInfo CALLED");
var receivedObj = new LoginInfo({
    username:username ,
    password:password
});
console.log(receivedObj);
receivedObj.save(function(err , data){
    console.log("inside Save "); 
    if(err){
        //res.send(err);
        console.log(err);
    }
    else{
        callback();   // the error is here
    }
});

}

please guide me what is the error in this function