Django：自定义表单错误消息

Question

class SignUpForm(UserCreationForm):


    class Meta():
        model = User
        fields = ('username', 'email', 'first_name', 'last_name')
        error_messages = {
            'last_name': {
                'required': _("Please enter your last name."),
            },
            'first_name': {
                'required': _("Please enter your first name."),
            },
            'password1': {
                'required': _("Please enter your first password"),
            },
             'password2': {
                 'required': _("Please enter your second password."),
            },
         }

为什么这种定制不起作用？此外，我不知道为什么发现电子邮件，first_name和last_name不是必填字段。