Delphi Access violation address - delphi

Question

procedure TWebModule2.ClientGetItem(Request: TWebRequest; Response: TWebResponse);
var
o: TJSONObject;
CfKey: string;
a:TjsonArray;
begin
CfKey:='';
CfKey:= Request.QueryFields.Values['CF'] ;
if Request.QueryFields.Count>0 then begin
QryClientParam.SQL.Text := 'select * from TABLE where COD_FISC = :CF';
// qryclient.Params.ParamByName('CF').AsString := Request.QueryFields.Values['CF'];
QryClientParam.Params.ParamByName('CF').AsString := CfKey;
end else begin
QryClientParam.SQL.Text := 'select * from TABLE';
end;
QryClientParam.Active := true;
if QryClientParam.Active then begin
if QryClientParam.RecordCount>0 then begin
QryClientParam.First;
o := TJSONObject.Create;
// o.AddPair('EmployeeNumber',TJSONNumber.Create( qryClient.FieldByName('COD').AsInteger ));
o.AddPair('Descrizione', QryClientParam.FieldByName('DESK').AsString);
o.AddPair('Codice Fiscale', QryClientParam.FieldByName('COD_FISC').AsString);
o.AddPair('CAP', QryClientParam.FieldByName('CAP01').AsString);
o.AddPair('Indirizzo', QryClientParam.FieldByName('INDIRI').AsString);
o.AddPair('Citta', QryClientParam.FieldByName('CITTA01').AsString);
a.AddElement(o);
QryClientParam.Next;
Response.ContentType := 'application/json';
Response.Content := a.ToString;
// TFile.WriteAllText('JasonResult'+ '.json', a.ToString); //Scrivo Array del risultato in file
end;
end;
end;
I get this error:
Access violation at address 00E9D93D in module 'WebServerClienti.exe'. Read of address 00000008
The error is before this line:
a.AddElement(o);
The procedure works fine with my total query without parameter.
I don't understand - please help me