InfoPath Dev

 There is a ASP.NET Web Service which uses SessionState. Let's see it's 2 methods:

        [WebMethod(EnableSession = true)]

        public string SetUser(string userName)

        {
            return UserName = userName;
        }

        [WebMethod(EnableSession = true)]
        public string GetUser()
        {
                return UserName;           
        }

        private string UserName
        {
            get
            {
                return Session[Consts.UserNameKey].ToString();
            }
            set
            {
                Session[Consts.UserNameKey] = value;
            }
        }

when I test these methods (SetUser, then GetUser) through browser via local ASP.NET Server and then on IIS, it works right.

I created browser-enabled InfoPath form and call SetUser with InfoPath function userName() parameter value and then call GetUser in Form Open rules set.

In preview mode it also works right, but when I deploy form into Office Forms Server, GetUser returns "Object reference not set to an instance of an object".

What I need to correct here?