How do I pass an object as a parameter from Delphi code to a Fastscript function?

Given a Fastscript function like this:

function IsObjValid(obj: TSomeObject): boolean;

begin

  Result := (obj.GetValue <> 0);

end;

I want to call this function from my Delphi code passing a TSomeObject instance. Therefore I register the TSomeObject class like this:

type

  TSomeObject = class end;

// ...

 cl := FScript.AddClass(TSomeObject, 'TObject');

  cl.AddMethod('function GetValue: integer', MethodSomeObjectGetValue);

... and call it like this:

  FCurrentObj := TStringList.Create;

  res := FScript.CallFunction('CheckInstance', Integer(FCurrentObj));

In MethodSomeObjectGetValue I expect Instance to be the value I passed as parameter above:

function TMyStript.MethodSomeObjectGetValue(Instance: TObject; ClassType: TClass;

const _MethodName: string; Caller: TfsMethodHelper): Variant;

begin

// function GetValue: integer

  Assert(Integer(_Instance) = Integer(FCurrentObj), 'invalid Instance value');

// .... more code

end;

But it is always NIL, so I must be doing something wrong, but what?