我有一個模型，當用戶在date_fields中放置回溯日期或輸入到今天的較早日期時，我想引發錯誤。class Leave_Management(models.Model):        employee = models.ForeignKey('employeeModel', on_delete=models.CASCADE)        name = models.CharField(max_length=50)        reason = models.TextField(max_length=200)        date_to = models.DateField(null=True)        date_from = models.DateField(null=True)表格class LeaveForm(forms.ModelForm):    class Meta:        model = Leave_Management        fields = ('__all__')    def clean_date_to(self):        date_to = self.cleaned_data.get('date_to')        if not date_to > datetime.date.today():            raise ValidationError('Enter Valid Date')        return date_to我嘗試編寫此驗證代碼，但顯示錯誤 TypeError at /home/leave'>' not supported between instances of 'NoneType' and 'datetime.date'print（date_to）僅顯示date 2018-06-2print(datetime.date.today()) 像這樣顯示 datetime.date(2018, 6, 20)我究竟做錯了什么？