Oracle External Proc / ORDCOM argument value not recognized

I have a COM Dll with the following interface

HRESULT CreateEvent(

                [in, out] long* timeout,

                [in, out] BSTR* area,

                [in, out] BSTR* FieldValues,

                [in, out] BSTR* EventDescription,

                [in, out] BSTR* ErrMsg,

                [in, out] long* ErrMsgSize,

                [out, retval] short* );

I use it from a VB6 application in the same machine and it works properly.

When I try to use it in PL/SQL with ORDCOM everything works correctly except the arguments are not recognized at the destiny process.

For instance if I set the area as

v_area VARCHAR2(2) := 'US';

...

ORDCOM.InitArg();

ORDCOM.SetArg(v_area,'pBSTR')

...

hresult := ORDCOM.invoke(...

The invoke method works and returns 0, but the value v_area is not received as 'US' at the other end.