error :cannot convert null to string در جدول ado

[northkh_rah]

 

while adotable1.Eof=false do

begin

s1:= adotable1.FieldValues['nam'].asstring;

s2:= adotable1.FieldValues['famil'].asstring;

s3:=adotable1.FieldValues['semat'].asstring;

s4:= adotable1.FieldValues['mohalkh'].asstring;

s5:= adotable1.FieldValues['shahr'].asstring;

s6:=adotable1.FieldValues['tell'].asstring;

s7:=adotable1.FieldValues['address'].asstring;

str1:=edit1.Text;

i:=pos(str1,s4);

label2.Caption:=inttostr(i);

if i<>0 then

adotable2.InsertRecord([s1,s2,s3,s4,s5,s6,s7]);

adotable1.Next;

end;

s1,s2, ...,str:stringهستند