django-allauth: Module "accounts.forms" does not define a "SignupForm" class

I am getting the following error:

django.core.exceptions.ImproperlyConfigured: Module "accounts.forms" does not define a "SignupForm" class

settings.py

(...)

ACCOUNT_SIGNUP_FORM_CLASS = 'accounts.forms.SignupForm'

(...)

accounts/forms.py

from allauth.account.forms import BaseSignupForm

class SignupForm(BaseSignupForm):

    def __init__(self, *args, **kwargs):
        self.sociallogin = kwargs.pop('sociallogin')
        user = self.sociallogin.account.user
        first_name = forms.CharField(label=_('First name'),
                                     max_length=30,
                                     min_length=2,
                                     widget=forms.TextInput(attrs={
                                         'placeholder':_('First name')}))
        last_name = forms.CharField(label=_('Last name'),
                                     max_length=30,
                                     min_length=2,
                                     widget=forms.TextInput(attrs={
                                         'placeholder':_('Last name')}))
        second_last_name = forms.CharField(label=_('Second last name'),
                                     max_length=30,
                                     empty='',
                                     widget=forms.TextInput(attrs={
                                         'placeholder':_('Second last name')}))
        # TODO: Should become more generic, not listing
        # a few fixed properties.
        initial = {'email': user_email(user) or '',
                   'username': user_username(user) or '',
                   'first_name': user_field(user, 'first_name') or '',
                   'last_name': user_field(user, 'last_name') or ''}
        kwargs.update({
            'initial': initial,
            'email_required': kwargs.get('email_required',
                                         app_settings.EMAIL_REQUIRED)})
        super(SignupForm, self).__init__(*args, **kwargs)

    def save(self, request):
        adapter = get_adapter()
        user = adapter.save_user(request, self.sociallogin, form=self)
        # TODO: Add request?
        super(SignupForm, self).save(user)
        return user

    def raise_duplicate_email_error(self):
        raise forms.ValidationError(
            _("An account already exists with this e-mail address."
              " Please sign in to that account first, then connect"
              " your %s account.")
            % self.sociallogin.account.get_provider().name)