Estoy intentando crear formularios de registro de usuario personalizados en Django pero obtengo el siguiente error. Todo en mi página se muestra correctamente, pero me sale el error.

Error:

Exception Type: KeyError
Exception Value:    'First name'

Mi formulario.py:

from django import forms            
from django.contrib.auth.models import User   # fill in custom user info then save it 
from django.contrib.auth.forms import UserCreationForm      

class MyRegistrationForm(UserCreationForm):
    email = forms.EmailField(required = True)
    first_name = forms.CharField(required = False)
    last_name = forms.CharField(required = False)
    birtday = forms.DateField(required = False)



    class Meta:
        model = User
        fields = ('username', 'email', 'password1', 'password2')        

    def save(self,commit = True):   
        user = super(MyRegistrationForm, self).save(commit = False)
        user.email = self.cleaned_data['email']
        user.first_name = self.cleaned_data['First name']
        user.last_name = self.cleaned_data['Last name']
        user.birthday = self.cleaned_data['Birthday']


        if commit:
            user.save()

        return user

Mis vistas.py

from django.shortcuts import render
from django.http import HttpResponseRedirect    
from django.contrib import auth                 
from django.core.context_processors import csrf 
from forms import MyRegistrationForm

def register_user(request):
    if request.method == 'POST':
        form = MyRegistrationForm(request.POST)     # create form object
        if form.is_valid():
            form.save()
            return HttpResponseRedirect('/accounts/register_success')
    args = {}
    args.update(csrf(request))
    args['form'] = MyRegistrationForm()
    print args
    return render(request, 'register.html', args)