Django create custom UserCreationForm

I enabled the user auth module in Django, however when I use UserCreationForm it only asks for username and the two password/password confirmation fields. I also want email and fullname fields, all set as required fields.

I've done this:

from django.contrib.auth.forms import UserCreationForm
from django import forms
from django.contrib.auth.models import User

class RegisterForm(UserCreationForm):
    email = forms.EmailField(label = "Email")
    fullname = forms.CharField(label = "Full name")

    class Meta:
        model = User
        fields = ("username", "fullname", "email", )

Now the form shows the new fields but it doesn't save them to the database.

How can I fix this?