Aspnetcore: HttpContext.Session.SetString

AspNet core rc2
package: Microsoft.AspNetCore.Session

    HttpContext.Session.SetString(key,value)   error.

I have add code in the startUp.cs.

        public void ConfigureServices(IServiceCollection services)
        {
            // Add framework services
            services.AddApplicationInsightsTelemetry(Configuration);
            services.AddAuthorization();
            services.AddSession(options =>
            {
                options.IdleTimeout = TimeSpan.FromDays(7);
                options.CookieName = ".FileSystem";
            });
            services.AddMvc();
        }

         public void Configure(IApplicationBuilder app, IHostingEnvironment env, ILoggerFactory loggerFactory)
        {
            #region

            loggerFactory.AddConsole(Configuration.GetSection("Logging"));
            loggerFactory.AddDebug();

            app.UseApplicationInsightsRequestTelemetry();

            if (env.IsDevelopment())
            {
                app.UseDeveloperExceptionPage();
                app.UseBrowserLink();
            }
            else
            {
                app.UseExceptionHandler("/Home/Error");
            }

            app.UseApplicationInsightsExceptionTelemetry();

            app.UseStaticFiles();

            app.UseMvc(routes =>
            {
                routes.MapRoute(
                    name: "default",
                    template: "{controller=Home}/{action=Index}/{id?}");
            });

            #endregion

            app.UseSession();
        }
    }

** Not working. HttpContext.session.setString()

error: Session has not been configured for this application or request.

I have read some blogs: asp.net core rc1.
The blogs shows that i should import the package: aspnet.session.
but when i import this package, the package can not support app.UseSession() function.

Help！ how can i use session in aspNet core RC2