类型参数“T”必须是类类型

以下是其中一个函数在helper类中的外观:

function RESTServiceCallHelper.ExecuteAsObject<TObj>(resource: string): TObj;
var
 RestRequest: TRESTRequest;
 obj: TObj;
begin
  PrepareRequest(RestRequest, resource);
  RestRequest.Execute;
  obj := TJson.JsonToObject<TObj>(RestRequest.Response.Content);
  result:=obj;
end;

   TPartnerCreditInfo   = class
      FlCreditClassDesc: string;
      FCppID: string;
      FCreditClass: string;
      FCreditClassDesc: string;
      FCreditLimit_CurrencyCode: string;
      FCreditLimit: double;
      FAmountBalance: double;
      FAmountBalance_CurrencyCode: string;
      FAmountBalanceLast: double;
      FAmountBalanceLast_CurrencyCode: string;
   end;

procedure TModuleX.CallAPIS;
var
  lRESTHelper : TrpRESTServiceCallHelper;
  pc: TPartnerCreditInfo;
begin
  lRESTHelper:= TrpRESTServiceCallHelper.Create('https://mydomain/api');
  lRESTHelper.AddQueryStringParam('param1','paramvalue');

  pc:=lRESTHelper.ExecuteAsObject<TPartnerCreditInfo>('resource');
  showmessage(pc.FCppID);
end;

我现在面临的问题是:

obj := TJson.JsonToObject<TObj>(RestRequest.Response.Content);
[dcc32 Error] RESTServiceCallObj.pas(99): E2511 Type parameter 'T' must be a class type

根据文档,jsontobject函数的T参数必须是一个类,tpartnercredinfo也是一个类。为什么tpartnercredinfo没有被识别?