Membership nao contem GetUser

Question

Este e' meu codigo:

if (System.Web.Security.Membership.GetUser() != null)
            {
                System.Web.Security.MembershipUser m = Membership.GetUser();

                try
                {

Qdo faco Build funciona. Quando faco um publish da esse erro:

Error 80 'Membership' does not contain a definition for 'GetUser' c:\Users\Me\AppData\Local\Temp\WebSitePublish\myproject-1834603012\obj\Debug\AspnetCompileMerge\Source\ShopCart.aspx.cs 54 0f

Por que ?