Python Django错误，管理员表单TypeError位于/admin/sys/pet_history/add/___str___________________________

我正在尝试创建django admin表单和所有其他组件，但是当我尝试在表单上添加到此类时，返回以下错误：

TypeError at /admin/syst/pet_history/add/
__str__ returned non-string (type NoneType)

有人知道为什么会发生这种情况吗？下面是该类的代码：

class Pet_History(models.Model):
    pet_history_ID = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False)
    pet_ID = models.ForeignKey(Pet_Description, on_delete=CASCADE)
    visit_Num = models.IntegerField(null=True)
    Visited_vet_before = models.CharField(max_length=50, null=True)
    First_time_of_illness = models.DateField(null=True)
    Special_Medication = models.CharField(max_length=50, null=True)
    Medication_Description = models.CharField(max_length=50,null=True)

    def __str__(self):
        return {self.visit_Num}