Integrity error: column username is not unique. Как исправить?

Есть формы регистрации и входа. После ввода данных и нажатий на кнопку появляется ошибка: "Integrity error: column username is not unique". Но у меня есть лишь email, username не использую. Что не так, как исправить?

Views:

def register(request):
    registered = False
    if request.method == "POST":
        user_form = UserForm(data=request.POST)
        if user_form.is_valid():
            user = user_form.save()
            user.set_password(user.password)
            user.save()
            registered = True
        else:
            print(user_form.errors)
    else:
        user_form = UserForm()
    return render(request, 'landing.html', {'user_form':user_form, 'registered':registered})


def user_login(request):
    if request.method == "POST":
        email = request.POST.get('email')
        password = request.POST.get('password')
        user = authenticate(email='email', password='password')
        if user:
            if user.is_active():
                login(request, user)
                return HttpResponseRedirect(reverse('materials'))
            else:
                return HttpResponse("Account is not active")
        else:
            return HttpResponse("Failed to login")
    else:
        return render(request, 'landing.html', {})

Forms:

class UserForm(forms.ModelForm):
    password = forms.CharField(widget=forms.PasswordInput())

    class Meta:
        model = User
        fields = ('email', 'password')