error_messages 'password1' 'required' can not be overidden

[Oskar Haller]

class UserRegisterForm(UserCreationForm):
    """
    Form class to register a new user
    """
    class Meta:
        #https://docs.djangoproject.com/en/2.1/topics/forms/modelforms/#modelforms-overriding-default-fields
        model = User
        fields = (...'password1',  'password2')
        error_messages = {
            'password1': {
                'required': _('Please enter a password.')
            },
            'password2': {
                'required': _('Please enter a password.')
            }
        }

Instead of 'Please enter a password' I get 'This field is required.'

For all other fields the overrriding works fine