我用ADOQuery与DbGrid显示表中的记录，为什么当表中的没有记录时就报错，“invalid variant type conversion”

我的代码如下：
 str1:='select Goods.G_Class as C_Code1,Goods.G_Code Code1 ,Goods.G_Name  GName1,Goods.G_GuiGe GuiGe1 ,Class.C_Name CName1,Goods.G_DanWei DanWei1,Goods.G_Min min1,Goods.G_Max max1,  ';
  str2:='round(Goods.G_Num,2) Num1,round(Goods.G_Jia,2) Jia1,round(Goods.G_Amount,2) Amount1 ';
str3:='from Goods,Class where Goods.G_Class=Class.C_Code and Class.C_Name='''+trim(cmtcom1.text)+'''';
end;
str1:=str1+str2+str3;
cmttable1.Close;
cmttable1.SQL.Clear;
cmttable1.SQL.Add(str1);
cmttable1.Open; Pdata.Classstr:='';
     Pdata.classstr:=cmttable1.fieldbyname('C_Code1').value;
end;
dbgrid1.Columns[0].FieldName:='Code1';
dbgrid1.Columns[1].FieldName:='GName1';
dbgrid1.Columns[2].FieldName:='GuiGe1';
dbgrid1.Columns[3].FieldName:='CName1';
dbgrid1.Columns[4].FieldName:='DanWei1';
dbgrid1.Columns[5].FieldName:='min1';
dbgrid1.Columns[6].FieldName:='max1';
dbgrid1.Columns[7].FieldName:='Num1';
dbgrid1.Columns[8].FieldName:='Jia1';
dbgrid1.Columns[9].FieldName:='Amount1';