django - タイプ 'NoneType'の引数は、django fromsで反復可能ではありません

Question

def clean(self):
      """ 
      Override the default clean method to check whether this course has been already inputted.
      """    
      cleaned_data = super(tbmstappraisalschedForm, self).clean()
      #appsched_id = str(self.cleaned_data.get('intAppSchedID'))
      depart_id = self.cleaned_data.get('intDeptID')
      fromdate = str(self.cleaned_data.get('sdtFromDate'))
      todate = str(self.cleaned_data.get('todate'))
      pk=self.instance.pk
      #if tbmstappraisalsched.objects.filter(intDeptID=depart_id).exclude(pk=self.instance.pk).exists():
      #qry = "SELECT intAppSchedID FROM tbMstAppraisalSched WHERE intDeptID ='"+depart_id+"' AND (('"+fromdate+"' BETWEEN  sdtFromDate AND  sdtToDate) OR ('"+todate+"' BETWEEN  sdtFromDate AND sdtToDate))"
      qry = """SELECT intAppSchedID FROM tbMstAppraisalSched WHERE intDeptID = '{}' AND (('{}' BETWEEN sdtFromDate AND sdtToDate) OR ('{}' BETWEEN sdtFromDate and sdtToDate))"""
      res = tbmstappraisalsched.objects.raw(qry.format(depart_id.pk, fromdate, todate))
      for re in res:
        if(re.intAppSchedID != pk):
            msg = "The slot for selected department and selected dates exists"
            raise ValidationError(msg)
        else:
            return self.cleaned_data

クエリに行が返されない場合、例外が発生します 例外タイプ: TypeError 例外値:
タイプ 'NoneType' の引数は反復可能ではありません よろしくお願いします