GetCurrentShoppingCart() throwing NullReferenceException

Getting an unexpected NullReferenceException from the ShoppingService.GetCurrentShoppingCart() method. Here's the relevant stacktrace:

[NullReferenceException: Object reference not set to an instance of an object.] CMS.Ecommerce.CurrentShoppingCartService.GetCurrentShoppingCart(UserInfo user, SiteInfoIdentifier site) +40 CMS.Ecommerce.ECommerceContext.get_CurrentShoppingCart() +231 Kentico.Ecommerce.ShoppingService.GetCurrentShoppingCart() +29 Sologic.Website.Current.get_Order() in C:\Sologic\src\sologic.website\Website\Current.cs:36 Sologic.Website.Filters.ViewBagFilter.OnActionExecuting(ActionExecutingContext filterContext) in C:\Sologic\src\sologic.website\Website\Filters\ViewBagFilter.cs:44

The code isn't exotic:

public static Lib.Services.Order Order
{
    get
    {
        if (HttpContext.Current.Session["Order"] == null)
            HttpContext.Current.Session["Order"] = new Lib.Services.Order(Culture, new ShoppingService().GetCurrentShoppingCart());

        return HttpContext.Current.Session["Order"] as Lib.Services.Order;
    }
}

It worked when the user already had a cart object somehow. Is there some cart initialization code that needs to run first? I think because this is called very early on (after application start and before any controller request has processed) there must be some initializer that didn't get a chance to fire. What needs to happen for this code to run successfully? I'd understood .GetCurrentShoppingCart() would create a cart if it didn't exist. There must be some precondition to that?