Error when authenticating a user

I have recently added this page to my website:

public class MasterMxPage : System.Web.UI.MasterPage
{
    private BLL.User _user;

    public BLL.User MxUser
    {
        get
        {
            if (Session["MxUser"] != null)
            {
                _user = (BLL.User)Session["MxUser"];
            }
            else if (Membership.GetUser() != null)
            {
         &n ...