How to add a new requirement with TestDirector API?

Hi,

I'm trying to connect to TD 8 by using the API with C# .NET. I get one probl
em: I cannot add requirement. I get exception "the parameter is incorrect" a
t reqFactory.AddItem(null).

Here is my code:

tdc = (TDConnection) Session["TDConnect"];
ReqFactory reqFactory = (ReqFactory) tdc.ReqFactory;		
Req req = (Req) reqFactory.AddItem(null);	

Have I done something wrong?