I ran into an error and I don't know why. I try to transform all the character into lowercase before inserting into database and for comparison purposes.
At the first function this.addUser I call toLower to transform it become lowercase it has no problem but at second function this.checkPublicUsername I call toLower it display an error in my terminal said that TypeError: Cannot call method 'toLowerCase' of undefined

function toLower (text) {

    return text.toLowerCase();

}



this.addUser = function(data,callback) {

    use strict;

    // Generate password hash

    var salt = bcrypt.genSaltSync();

    var password_hash = bcrypt.hashSync(data.password, salt);

    // transform all character into lowercase

    var real = toLower(data.publicUsername);

    // Create user document

    var user = {

        '_id': data.email,

        'email':data.email,

        'password': password_hash,

        'name':{

            'firstName': data.firstName,

            'lastName': data.lastName

        },

        'penName': data.penName,

        'publicUsername':{

            'display':data.publicUsername,

            'real':real

        }

    };

    users.insert(user, function (err, result) {

        use strict;



        if (!err) {

            console.log(Inserted new user);

            return callback(null, result[0]);

        }

        if (err) console.log(err)



        return callback(err, null);

    });

}



this.checkPublicUsername = function(data,callback) {

    use strict;

    var real = toLower(data);

    var publicUsername = {

        publicUsername.real:real

    }

    users.find(publicUsername).count(function(err, result) {

        callback(null,result);

    })



}

call checkPublicUsername

 users.checkPublicUsername(data.publicUsername, function(err, result){

     if(result){

     callback(new Error(''+data.publicUsername+' has been taken. Please choose another.'), null);

             }

        });