Problem inserting data into firebird table

Hi, I have a problem inserting data into Firebird tables.

- Connecting to database: no problem
- Using TIBDatabase; TIBTransaction, TIBQuery
- Running first TIBQuery on table 'STRAAT'  1 field with Unique index 'STRATEN'
   
  IBQuery1.Close;              //Straat
  IBQuery1.Prepare;
  IBQuery1.Open;
  IBQuery1.First;
   for i:= 0 to IBQuery1.RecordCount-1 do begin
    ComboBox2.Items.Add(IBQuery1Straten.Value);
    IBQuery1.Next;
   end;

Then by exit ComboBox2

procedure TAdministratie.ComboBox2Exit(Sender: TObject);
var A: string;
begin
with Datamodule1 do begin
 if Trim(ComboBox2.Text) = '' then begin
  Exit;
 end
  else
   begin
    A:= Trim(ComboBox2.Text);
    if not IBQuery1.Locate('Straten', Trim(ComboBox2.Text), [])then begin
      IBQuery5.Close;
      IBQuery5.SQL.Clear;
      IBQuery5.SQL.Add('Insert INTO STRAAT(STRATEN) Values(:A)');
      IBQuery5.Prepare;
      IBQuery5.ExecSQL;
    end
     else
      begin
       Exit;
      end;
  end;
end;
end;

Result is that in the field of the table, data: NULL, instead of the naam of ComboBox2

what might be the problem here ? I know something about SQL-programming, but connot get this right. Can anyone help me on this

thx
Mario