Ember data post form data as JSON

Common feature - register user - store data in mongodb. In postman everything is ok, I can't make it good from app ui although.

My controller:

export default Ember.Controller.extend({
actions: {
    addUser: function(){
        let formData = {
            "fullname": this.get('fullname'),
            "firstname": this.get('firstname'),
            "lastname": this.get('lastname'),
            "email": this.get('email')
        };

        let newUser = this.store.createRecord('candidate',{
            fullname: formData.fullname,
            firstname: formData.firstname,
            lastname: formData.lastname,
            email: formData.email
        })
        newUser.save();

    }
}
});

My adapter:

import DS from 'ember-data';

export default DS.RESTAdapter.extend({
    namespace: 'api'
});

My model:

import DS from 'ember-data';

export default DS.Model.extend({
  fullname: DS.attr('string'),
  firstname: DS.attr('string'),
  lastname: DS.attr('string'),
  email: DS.attr('string')
});

post request is successful with 201 status. In db I see new empty document. In console I see error "SyntaxError: Unexpected token C in JSON at position 0" I guess I should have declare JSON.parse. In adapter? Making JSON.parse(newUser) doesn't help. I am newbie with ember so I guess I silly mistake but I cannot find any.