无法将日期字段更新为null

Question

不能插入并将日期字段更新为null

  

err
  [“''值的日期格式无效。它必须为YYYY-MM-DD格式。”]

`class StudentUserMedicalRecord(models.Model):
     ImmnunBCGDateFirstDose=models.DateField(null=True, blank=True)
     ImmnunBCGDateSecondDose=models.DateField(null=True, blank=True)
     ImmununBCGDateThirdDose=models.DateField(null=True, blank=True)
     ImmnunBCGDateBooster1=models.DateField(null=True, blank=True)
     ImmnunBCGDateBooster2 = models.DateField(null=True, blank=True)
    def __str__(self):
       suser = '{0.Student_Users}'
    return suser.format(self)
 //my html
     <td><input type='date' name='firstdate'  /></td>
     <td><input type='date' name='seconddate'  /></td>
     <td><input type='date' name='thirddate'  /></td>
     <td><input type='date' name='firstbooster'  /></td>
     <td><input type='date' name='secondbooster'  /></td>
//my views
     first = request.POST['firstdate']
     second = request.POST['seconddate']
     third = request.POST['thirddate']
     booster = request.POST['firstbooster']
     secondbooster = request.POST['secondbooster']
       V_insert_data = StudentUserMedicalRecord(
          ImmnunBCGDateFirstDose=first,
          ImmnunBCGDateSecondDose=second,
          ImmununBCGDateThirdDose=third,
          ImmnunBCGDateBooster1=booster,
          ImmnunBCGDateBooster2=secondbooster,
     )
      V_insert_data.save()